Preheat your oven to 350°F (175°C). Grease a 9-inch (23 cm) round cake pan and line the bottom with parchment paper for easy release. Set aside.

In a medium bowl, whisk together the flour, cocoa powder, baking powder, and salt until fully combined. Set aside.

In a large mixing bowl, beat the butter and sugar using an electric mixer on medium speed for about 3–4 minutes, until light, pale, and fluffy. Scrape down the sides of the bowl as needed to ensure even mixing.

Add the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract until smooth.

Add the dry ingredients to the butter mixture in three parts, alternating with the milk (beginning and ending with the dry ingredients). Mix until just combined — the batter will be thick.

Carefully stir in the boiling water. The batter will thin out and become pourable — this is normal and expected.

Pour the batter into the prepared cake pan and smooth the top.

Bake for 25–30 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs attached.

Let the cake cool completely in the pan before transferring it to a wire rack.

(Pro Tip: Need to speed it up? Place the cake in the fridge for about 10–15 minutes.)

Place the chopped chocolate in a microwave-safe bowl and heat in 30-second bursts, stirring between each, until melted and smooth. Alternatively, use a double boiler. Set aside to cool slightly — it should be warm, not hot.

In a separate bowl, whisk the egg yolks and sugar together until thick, pale, and creamy — about 2 minutes.

In a small saucepan, sprinkle the gelatin powder over cold water. Let it bloom for 1–2 minutes, then gently heat over low heat, stirring until completely dissolved. Remove from heat.

Slowly mix the melted chocolate into the egg mixture, stirring constantly until smooth.

Stir the dissolved gelatin into the chocolate and egg mixture until fully incorporated.

In another bowl, whip the heavy cream and vanilla extract until stiff peaks form (about 3–4 minutes).

Add a small scoop of whipped cream to the chocolate mixture to lighten it, then gently fold in the remaining whipped cream.

(Pro Tip: Fold slowly and carefully to keep the mousse airy and fluffy.)

Once fully cooled, carefully slice the cake in half horizontally using a serrated knife or cake leveler to create two even layers.

Gently place the second cake layer on top, aligning it carefully.

Use the remaining chocolate mousse to frost the top and sides of the cake. Smooth it out with an offset spatula or create rustic swirls and peaks for a homemade look.

(Tip: If the mousse is too soft to spread neatly, refrigerate it for 10–15 minutes to firm up.)

For extra indulgence, melt equal parts heavy cream and chopped chocolate until smooth, then pour the ganache over the top of the cake. Let it drip naturally down the sides or spread it evenly for full coverage.

Refrigerate the cake for at least 2 hours (or longer) to allow the mousse and ganache to set completely.

Garnish the top of the cake with chocolate shavings, fresh berries, a light dusting of cocoa powder, or powdered sugar right before serving.