1. Make the Chocolate Cake Layer:

• Preheat the oven to 350°F (175°C). Grease a 9-inch regular round baking pan (NOT springform yet) and line the bottom with parchment paper.
• In a large bowl, stir or whisk together the dry cake ingredients: sugar, flour, cocoa, baking powder, baking soda, and salt.
• In a separate medium bowl, whisk the egg, milk, oil, and vanilla.
• Add the wet ingredients to the dry ingredients and mix on medium speed (or by hand) for 2 minutes until combined.
• Carefully stir in the boiling water until the batter is smooth (it will be thin).
• Pour the batter into the prepared 9-inch round pan.
• Bake for 25-35 minutes, or until a toothpick inserted into the center comes out clean.
• Cool the cake in the pan for 10 minutes, then invert it onto a wire rack upside down (this helps flatten the top). Peel off the parchment paper and let cool completely.  

2. Prepare the Mousse Components:

• Bloom Gelatin: In a small dish, sprinkle the 2 teaspoons of unflavored gelatin over the 2.5 tablespoons of cool water. Let it sit for 5-10 minutes to soften and bloom.
• Melt Chocolates & Butter: In a heat-proof bowl set over a saucepan of simmering water (double boiler) or in the microwave (using short bursts, stirring often), combine the chopped semi-sweet chocolate, white chocolate, and butter. Melt until smooth. Stir well and set aside to cool slightly (to lukewarm).
• Heat Gelatin: Gently heat the softened gelatin over low heat (or microwave 5-10 seconds) just until it dissolves and becomes completely liquid and lump-free. Let it cool slightly so it’s not hot.
• Whip Cream: In a large, chilled bowl, beat the 1 3/4 cups cold heavy cream and 1 1/2 teaspoons vanilla extract with an electric mixer until medium peaks form. Gradually add the powdered sugar and continue beating until stiff peaks form.
• Incorporate Gelatin into Cream: With the mixer on low speed, slowly drizzle the slightly cooled, liquid gelatin into the whipped cream. Mix briefly until just combined.

3. Finish the Chocolate Mousse:

• Stir a few large spoonfuls of the stabilized whipped cream into the slightly cooled melted chocolate mixture to lighten it.
• Gently transfer the lightened chocolate mixture into the bowl with the remaining stabilized whipped cream.
• Using a large rubber spatula, gently fold the chocolate mixture into the whipped cream, scraping from the bottom up, until just combined and no streaks remain. Be careful not to deflate the mousse.

4. Prepare Strawberries and Assemble Cake:

• Place the completely cooled chocolate cake layer onto your serving plate. Secure the ring (sides only, no bottom) of a 9×3 inch springform pan tightly around the cake layer.
• Spread a thin layer of the chocolate mousse (about ½ cup) over the cake base inside the ring.
• Wash and hull the strawberries. Cut the stem ends flat. Select enough strawberries to stand halved, cut-side-out, tightly around the inside edge of the springform ring, pressing them gently into the mousse layer.
• Fill the center area completely with whole strawberries, standing them upright. Trim bottoms further if needed so all berries are roughly the same height, just below the top of the ring.
• Fill Gaps: Transfer the remaining mousse to a piping bag (optional, but easier) or use a spoon. Carefully fill in all the gaps between the strawberries with mousse, ensuring no air pockets remain.
• Spread the rest of the mousse evenly over the top of the strawberries, smoothing the surface right up to the top of the springform ring.

5. Chill the Mousse Cake:

• Place the assembled cake in the refrigerator for at least 4-6 hours, or preferably overnight, until the mousse is completely set and firm.

6. Make and Apply Ganache:

• Once the mousse is set, make the ganache. Place the 5.5 oz chopped semisweet chocolate in a heatproof bowl.
• Heat the 2/3 cup heavy cream in a small saucepan or microwave until simmering.
• Pour the hot cream over the chocolate. Let sit for 2-3 minutes.
• Gently whisk until the ganache is completely smooth. Let it cool slightly until it’s thick enough to spread but still pourable.
• Pour the ganache over the top of the chilled mousse cake (still in the ring or carefully remove ring first – see Q&A). Spread evenly to the edges.
• Place back in the fridge until the ganache has set (at least 30 minutes).

7. Make Chocolate Curls (Optional):

• Melt the 3.5 oz semisweet baking chocolate with the 1 tablespoon vegetable shortening or oil until smooth.
• Spread this mixture in a very thin, even layer onto the back of a cool metal baking sheet.
• Place the baking sheet in the freezer for 6-8 minutes, just until the chocolate is firm but not rock solid.
• Remove from the freezer and let it sit at room temperature for 1-2 minutes to slightly soften.
• Using a firm metal spatula held at a low angle, push the spatula firmly across the chocolate layer to create curls. (This takes practice! If it shatters, it’s too cold; wait longer. If it mushes, it’s too soft; chill briefly again).

8. Final Decoration and Serving:

• Carefully run a thin knife around the inside edge of the springform ring and release/remove the ring.
• Gently arrange the chocolate curls on top of the set ganache.
• Garnish with additional fresh strawberries if desired.
• Slice with a large sharp knife dipped in hot water and wiped clean between cuts. Serve chilled. Store leftovers in the fridge.