Step 1: Prepare the Chocolate Cake

1. Preheat the oven to 350°F (176°C). Line the bottom of an 8-inch cake pan with parchment paper and grease the sides.
2. In a large mixing bowl, cream together the butter and granulated sugar until light and fluffy (about 3–4 minutes).
3. Add the sour cream and vanilla extract, mixing until well combined.
4. Incorporate the egg whites, mixing until fully combined.
5. In a separate bowl, whisk together the flour, cocoa powder, baking powder, and salt.
6. Gradually add the dry ingredients to the butter mixture, alternating with the milk and water, beginning and ending with the dry ingredients. Mix until just combined.
7. Pour the batter into the prepared pan and bake for 18–20 minutes, or until a toothpick inserted into the center comes out clean.
8. Allow the cake to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.

Step 2: Prepare the Chocolate Fudge

1. Place the chocolate chips, corn syrup, and vanilla extract in a heatproof bowl.
2. Heat the heavy whipping cream until it just begins to boil, then pour it over the chocolate mixture. Let it sit for 2–3 minutes.
3. Whisk until the mixture is smooth and well combined. Set aside to cool slightly.

Step 3: Assemble the Cake Base

1. Once the cake has cooled, place it back into the 8-inch cake pan, lining the sides with parchment paper or a cake collar to extend the height.
2. Pour the chocolate fudge over the cake layer, spreading it evenly. Refrigerate until set.

Step 4: Prepare the Chocolate Mousse

1. In a heatproof bowl, whisk together the egg yolks, granulated sugar, and ½ cup (120ml) of heavy whipping cream.
2. Place the bowl over a pot of simmering water (double boiler method) and cook, whisking constantly, until the mixture reaches 160°F (71°C) and thickens. Remove from heat.
3. Add the semi-sweet chocolate chips to the egg mixture, stirring until smooth. Allow to cool to room temperature.
4. In a separate bowl, whip the remaining 1 ¼ cups (300ml) of heavy whipping cream with the powdered sugar until stiff peaks form.
5. Gently fold the whipped cream into the cooled chocolate mixture until fully combined.
6. Spread the chocolate mousse over the set fudge layer. Refrigerate until firm.

Step 5: Prepare the White Chocolate Mousse

1. Sprinkle the gelatin over the water in a small bowl and let it sit for about 5 minutes to bloom.
2. Melt the white chocolate chips in a heatproof bowl over simmering water (or microwave in short intervals, stirring until smooth).
3. Heat the bloomed gelatin until melted and smooth, then stir it into the melted white chocolate. Allow to cool slightly.
4. In a separate bowl, whip the heavy whipping cream and powdered sugar until stiff peaks form.
5. Gently fold the whipped cream into the white chocolate mixture until fully combined.
6. Spread the white chocolate mousse over the set chocolate mousse layer. Refrigerate for at least 4 hours, or until fully set.