Step 1: Make the Chocolate Cake Layers

1. Prepare the Oven and Pans: Preheat your oven to 350°F (175°C). Grease and line the bottoms of two 9-inch round cake pans with parchment paper.
2. Mix the Dry Ingredients: In a large bowl, sift together the flour, cocoa powder, baking soda, baking powder, and salt. Set aside.
3. Combine Wet Ingredients: In another large bowl, whisk together the sugar, vegetable oil, eggs, buttermilk, and vanilla extract until smooth.
4. Add the Dry Ingredients: Gradually add the dry ingredients to the wet mixture, stirring just until combined.
5. Add Hot Liquid: Slowly pour in the hot water or coffee, stirring until the batter is smooth. The batter will be thin—this is normal.
6. Bake the Cakes: Divide the batter evenly between the prepared pans and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
7. Cool the Cakes: Let the cakes cool in the pans for 10 minutes, then transfer them to a wire rack to cool completely.

Step 2: Prepare the White Chocolate Mousse

1. Melt the White Chocolate: Place the chopped white chocolate in a heatproof bowl. Heat ½ cup of heavy cream in a small saucepan over medium heat until it just begins to simmer. Pour the hot cream over the white chocolate and let it sit for 2-3 minutes. Stir until smooth, then set aside to cool slightly.
2. Bloom the Gelatin: In a small bowl, sprinkle the gelatin over 2 tablespoons of cold water. Let it sit for 5 minutes to bloom. Microwave the gelatin for 10 seconds to dissolve, then stir into the melted white chocolate mixture.
3. Whip the Cream: In a separate bowl, whip the remaining 1 cup of heavy cream and vanilla extract to soft peaks. Gently fold the whipped cream into the white chocolate mixture in three additions, being careful not to deflate the mousse. Cover and refrigerate for at least 1 hour to firm up.

Step 3: Make the Dark Chocolate Ganache

1. Heat the Cream: In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer.
2. Combine with Chocolate: Place the chopped dark chocolate in a heatproof bowl. Pour the hot cream over the chocolate and let it sit for 2-3 minutes. Add the butter and stir until the ganache is smooth and glossy. Let it cool slightly to a spreadable consistency.

Step 4: Assemble the Cake

1. Level the Cake Layers: Use a serrated knife to trim the tops of the cakes if they are domed, creating even layers.
2. Add the Mousse Layer: Place one cake layer on a serving plate or cake board. Spread the white chocolate mousse evenly over the top, smoothing it with an offset spatula.
3. Top with Second Layer: Place the second cake layer on top of the mousse, pressing gently to secure it.
4. Spread Ganache: Use the dark chocolate ganache to frost the top and sides of the cake. Smooth the ganache with a spatula or cake scraper for a polished finish. Refrigerate for 1 hour to set.

Step 5: Prepare the Chocolate Glaze

1. Heat the Cream: In a small saucepan, heat the heavy cream and corn syrup until it just begins to simmer.
2. Combine with Chocolate: Place the chopped chocolate in a heatproof bowl. Pour the hot cream mixture over the chocolate and stir until smooth. Let the glaze cool slightly until it thickens to a pourable consistency.

Step 6: Glaze the Cake

1. Apply the Glaze: Pour the chocolate glaze over the top of the chilled cake, letting it drip down the sides for a dramatic effect. Use a spatula to smooth the top if needed.
2. Decorate: Garnish the cake with chocolate curls, fresh berries, or powdered sugar for an elegant touch.

Step 7: Serve and Enjoy

Refrigerate the assembled cake for at least 2 hours (or overnight) to allow the layers to set. Before serving, let the cake sit at room temperature for 20-30 minutes to soften slightly. Slice and serve, and enjoy every decadent bite!