1. Prepare Oven and Pans:

• Heat oven to 350°F (175°C). For dark or nonstick pans, reduce temperature to 325°F (160°C).
• Spray the bottoms and sides of three 8-inch round cake pans with cooking spray. Line the bottoms of the pans with circles of cooking parchment paper. Set aside.

2. Make the Royal Blue Velvet Cake Batter:

• In a large mixing bowl, place the dry white cake mix, buttermilk, vegetable oil, eggs, unsweetened baking cocoa, royal blue paste food color, and the toothpickful of violet paste food color. (Add 1 tsp vanilla extract here if desired for cake flavor).
• Beat with an electric mixer on medium speed for 2 minutes, scraping the bowl occasionally with a rubber spatula to ensure all ingredients are well combined and the color is uniform.
• Divide the prepared batter evenly among the three lined cake pans. Smooth the tops.

3. Bake the Cake Layers:

• Place the pans in the preheated oven.
• Bake for 18 to 22 minutes, or until a toothpick inserted into the center of each cake layer comes out clean.
• Let the cakes cool in the pans on a wire cooling rack for 10 minutes.
• Carefully run a thin knife around the edges of the pans if needed, then invert the cake layers onto the cooling rack. Remove the parchment paper.
• Let the layers cool completely, about 45 minutes to an hour, before frosting.

4. Make the Marshmallow Creme Frosting:

• Ensure the butter is properly softened to room temperature.
• In a large bowl, using an electric mixer (stand mixer with paddle or handheld), beat the marshmallow creme and the softened butter together on medium speed until completely smooth and well combined. Scrape the bowl occasionally.
• Reduce the mixer speed to low. Gradually add the sifted powdered sugar and the salt.
• Beat on low speed until the sugar and salt are incorporated, then increase the speed to medium-high and beat until the frosting is smooth and fluffy.

5. Assemble and Frost the Cake:

• If necessary, use a long serrated knife to trim the rounded tops of two of the cake layers to create flat, even surfaces for stacking. Leave one layer untrimmed for the top.
• Place one of the trimmed cake layers, cut side up (or top side down), on your serving plate or cake stand.
• Spread about 1/3 cup (or a little more) of the marshmallow creme frosting evenly over this first layer.
• Top with the second trimmed cake layer, cut side up (or top side down). Spread with another 1/3 cup (or more) of frosting.
• Place the final, untrimmed cake layer on top, with its natural top side facing up.
• Frost the entire side and the top of the cake smoothly or decoratively with the remaining marshmallow creme frosting using an offset spatula or icing smoother.

6. Store:

• Store the finished Royal Blue Velvet Cake loosely covered in the refrigerator.
• For best flavor and texture, let slices sit at room temperature for about 15-20 minutes before serving.