Make the Chocolate Cake Layers:
Preheat oven to 350°F (180°C). Line two 8-inch round cake pans with parchment paper and spray with cake release or brush with butter.
Sift together flour, sugar, cocoa powder, baking powder, baking soda, and salt into a large bowl. Whisk until evenly combined.
In a medium bowl, combine eggs, milk, sour cream, vegetable oil, and vanilla. Whisk until smooth and homogenous. Ensure all ingredients are at room temperature.
Pour wet ingredients into dry ingredients. Whisk slowly until combined, scraping the bottom to catch any flour. Mix just until no lumps remain. Don't over-whisk.
Combine hot water with espresso powder in a small bowl. Stir until dissolved. Pour into cake batter and whisk gently until just combined. The batter will be thin.
Divide batter evenly between prepared pans. Bake for 35 minutes or until a skewer inserted in center comes out clean. Cool in pans for 1 hour, then transfer to wire rack to cool completely.
Make the Chocolate Mousse Layers:
Chop dark chocolate and white chocolate into fine pieces. Place in two separate bowls and set aside.
In a small bowl, whisk egg yolks, sugar, and cornstarch for 40 seconds until slightly thick and pale. Set aside.
Pour 1 cup heavy cream and vanilla into a saucepan. Warm over medium-low heat until just starting to heat up. Don't let it boil.
Drizzle a small amount of warm cream into beaten egg yolks while whisking constantly. Continue drizzling and whisking, tempering the eggs gradually.
Once all cream is whisked into egg yolks, pour everything back into saucepan. Heat over medium-low, stirring constantly until mixture thickens slightly. Remove from heat immediately.
Pour half of custard mixture into bowl with dark chocolate and half into bowl with white chocolate. Mix each until chocolate melts completely and incorporates smoothly.
In a separate bowl, whip remaining 1 cup heavy cream until stiff peaks form. Divide whipped cream evenly between both chocolate mousses. Fold in gently with spatula until combined.
Assemble the Tuxedo Cake:
Place a dessert ring (or springform pan with bottom removed) on a cake board or serving plate. Adjust to match cake size.
Cut any domed tops off cooled cake layers to make them flat. Trim to desired thickness, ensuring both layers match in height.
Place one chocolate cake layer inside dessert ring. Pour dark chocolate mousse on top and spread evenly.
Pour white chocolate mousse on top of dark chocolate layer. If dark mousse seems liquid, refrigerate 5 minutes first. Spread white mousse evenly.
Refrigerate three-layered cake for 30 minutes. Then carefully place second chocolate cake layer on top, pressing gently. Return to fridge and chill for at least 4 hours or overnight.
Decorate the Tuxedo Cake:
Chop milk chocolate into fine pieces. Heat ⅓ cup cream in small saucepan over medium heat until tiny bubbles form. Remove from heat.
Pour hot cream over chopped chocolate. Let sit 1 minute, then stir until chocolate melts completely and ganache is glossy and smooth.
Let ganache cool and thicken for about 30 minutes, until thick enough not to run off sides but thin enough to spread smoothly.
Remove cake from fridge and carefully unmold from cake ring. Use spoon or offset spatula to spread ganache on top, right to the edge.
Top with sprinkles, white chocolate curls, or chocolate shavings. Slice and serve chilled.