1. Make the Funfetti Cake Layers:

• Preheat oven to 340°F (170°C). Butter and flour three 6-inch round cake pans. Line bottoms with parchment paper. Apply cake strips if using.
• Sift the flour, granulated sugar, baking soda, baking powder, and salt together into a large bowl (or the bowl of your stand mixer). Whisk to combine.
• In a separate medium bowl, whisk together the room temperature egg whites, vanilla extract, room temperature sour cream, and room temperature milk.
• Add the wet ingredients to the dry ingredients. Mix on low speed until just combined. Do not overmix.
• Gently fold in the 1/3-1/2 cup of sprinkles with a spatula, mixing as little as possible.
• Divide the batter evenly among the prepared pans (about 370g each if weighing). Smooth the tops.
• Bake for 30-35 minutes, or until the centers are springy to the touch and a skewer inserted comes out clean.
• Let cool in the pans on wire racks for 20 minutes. Run a thin knife around the edges and carefully invert cakes onto racks. Peel off parchment and let cool completely.

2. Make the Italian Meringue Buttercream (IMBC):

• Ensure stand mixer bowl and whisk are meticulously clean and grease-free.
• Place the 5 room temperature egg whites, 2 dashes of cream of tartar, a pinch of salt, and 1/3 cup of granulated sugar into the mixer bowl.
• In a medium saucepan, combine the remaining 1 cup of granulated sugar and 1/3 cup of water. Place over low heat and stir constantly until the sugar completely dissolves and the syrup is clear. Stop stirring.
• Increase heat to medium-high. Clip on a candy thermometer. Bring the syrup to a boil and cook until it reaches 235-240°F (113-115°C – Soft Ball Stage).
• While the syrup cooks, start whipping the egg white mixture on medium-high speed until soft peaks form.
• Once the syrup reaches the target temperature, immediately remove it from the heat. With the mixer running on medium-low speed, very carefully and slowly drizzle the hot syrup down the side of the bowl into the whipping egg whites, avoiding the whisk.
• Once all syrup is added, increase the speed to high and whip until the meringue is stiff, glossy, and the outside of the bowl feels completely cool to the touch (this can take 10-15 minutes).
• Switch to the paddle attachment. With the mixer on medium-low speed, add the room temperature butter, one cube at a time, mixing well after each addition. The mixture might look curdled initially – this is normal! Keep beating on medium/medium-high speed, and it will eventually come together into a silky smooth buttercream.
• Beat in the 2 tsp vanilla extract. Set aside (keep at cool room temperature).

3. Make the American Funfetti Buttercream:

• In the clean bowl of a stand mixer with the paddle attachment (or using a hand mixer), beat the 1 lb room temperature butter until light and fluffy.
• Gradually add the sifted confectioners’ sugar on low speed until incorporated. Add the salt, 1 tsp milk, and 3 tsp vanilla extract. Mix until combined.
• Increase speed to medium-high and beat until light and fluffy.
• Divide the buttercream into two batches. Add 1 drop of pink food coloring to one batch and mix until the desired color is reached.
• Gently fold 1/3 cup sprinkles total into both batches (or add more to taste).
• Transfer the buttercream into piping bags: Put some pink in one bag, some white in another. Optionally, place one pink and one white bag together inside a larger piping bag fitted with a desired tip (or just snip the end) for the filling layer swirl effect mentioned.

4. Make the Pink Ganache:

• Finely chop the candy melts (if not already small). Place candy melts and white chocolate chips in a microwave-safe bowl.
• Add the heavy cream. Microwave on medium power (50%) in 20-30 second intervals, stirring well after each, until melted and smooth. Be careful not to overheat.
• Whisk until completely smooth and no lumps remain.
• Stir in 1 drop of pink food coloring, if needed for desired shade.
• Let the ganache sit and cool slightly for a couple of minutes until it’s thick enough for dripping but still pourable. Transfer to a piping bag or squeeze bottle for easier dripping.

5. Assemble the Cake:

• If necessary, level the tops of the completely cooled cake layers with a serrated knife.
• Place the first cake layer on your serving plate or cake stand.
• Pipe a border of the pink/white American Funfetti buttercream around the edge. Fill the center with more buttercream (or alternate colors). Smooth gently. (Recipe implies using the combined pink/white bag here for filling).
• Place the second cake layer on top. Repeat piping the buttercream border and filling.
• Place the third cake layer on top, upside down for a flat surface.
• Crumb Coat: Apply a thin layer of the Italian Meringue Buttercream over the entire cake (top and sides). Chill for about 10 minutes to set. (Recipe implies alternating pink/white ABC for crumb coat, but finishing with IMBC is more standard – adjust as desired based on interpretation).
• Final Frosting: Apply the remaining Italian Meringue Buttercream smoothly over the top and sides of the chilled cake using an offset spatula or bench scraper.
• Chill the frosted cake for another 10 minutes.

6. Decorate:

• Gently press confetti sprinkles onto the bottom 1-2 inches of the cake sides to create a “skirt.”
• Carefully apply the slightly cooled pink ganache around the top edge of the cake, encouraging drips down the sides. Smooth the ganache over the top surface if desired. Let the ganache set slightly (chill briefly if needed).
• Using a piping bag fitted with an 869 star tip (or similar large star tip), pipe dollops of the remaining Italian Meringue Buttercream around the top edge of the cake over the ganache.
• Chill briefly before serving.