White Chocolate Mocha Cake:
Prepare the Milk Mixture: Heat the milk and white chocolate together until melted and smooth. Stir in the instant espresso powder until fully dissolved. Allow the mixture to cool to room temperature.
Preheat Oven and Prepare Pans: Preheat your oven to 350°F (175°C). Grease and flour three 6-inch round cake pans, then line the bottoms with parchment paper.
Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
Cream Butter and Sugar: In a stand mixer fitted with a paddle attachment, cream together the butter and sugar on medium-high speed until pale and fluffy, about 3 minutes. Reduce speed to low, then add the eggs one at a time, mixing fully after each addition. Add the vanilla extract and mix until combined.
Combine Wet and Dry Ingredients: Alternate adding the flour mixture and the milk mixture to the butter mixture, beginning and ending with the flour (4 additions of flour, 3 of milk). Mix until just combined.
Bake the Cakes: Divide the batter evenly between the prepared cake pans. Bake for about 40 minutes, or until a toothpick inserted into the center comes out mostly clean. Cool the cakes in the pans on a wire rack for 10 minutes, then remove from the pans and let them cool completely on the rack.
Espresso Swiss Meringue Buttercream:
Prepare the Meringue: In the bowl of a stand mixer, whisk together the egg whites and sugar. Place the bowl over a double boiler and whisk constantly until the mixture reaches 160°F (70°C) and is no longer grainy to the touch.
Whip the Meringue: Transfer the bowl to the stand mixer and whisk on medium-high speed until the meringue is stiff and the bowl is no longer warm to the touch, about 5-10 minutes.
Add Butter and Espresso: Switch to the paddle attachment. Gradually add the cubed butter, mixing on low speed until smooth. Add the espresso powder and mix until fully incorporated and smooth.
White Chocolate Ganache:
Make the Ganache: Place the chopped white chocolate and heavy cream in a microwave-safe bowl. Microwave in 5-10 second increments, stirring between each interval, until the mixture is smooth and combined. Allow the ganache to cool and thicken.
Whip the Ganache: Remove 2/3 cup of the thickened ganache and set it aside. Whip the remaining ganache until light and fluffy.
Assembly:
Layer the Cake: Place one layer of the cooled cake on a cake stand or serving plate. Spread half of the whipped ganache evenly over the top. Repeat with the second layer. Place the final layer on top and apply a thin crumb coat of the remaining ganache around the cake. Chill for 20 minutes until set.
Frost the Cake: Frost the sides and top of the cake with the espresso Swiss meringue buttercream. Reserve some buttercream for the decorative border. Chill the cake for 30 minutes to firm up the frosting.
Apply the White Chocolate Drip: If necessary, gently warm the reserved 2/3 cup of ganache in the microwave for a few seconds to thin it out. Optionally, add a few drops of bright white color gel for a more vibrant look. Using a small spoon, drizzle the ganache around the edges of the chilled cake, allowing it to drip down the sides. Spread more ganache over the top of the cake and smooth with an offset spatula. Chill the cake for 5 minutes to set the ganache.
Decorate: Pipe a rope border on the top of the cake using a Wilton 1M tip with the reserved buttercream. Decorate the top with white nonpareils, dark chocolate crispearls, and coffee beans. Optionally, sprinkle some small nonpareils around the sides of the cake.