Begin by melting the white chocolate with milk in a medium saucepan over low heat, stirring until smooth. Preheat your oven to 350°F (175°C) and grease three 6-inch round cake pans with butter and flour.

In a mixing bowl, whisk together the all-purpose flour, baking powder, and salt. In a separate bowl, cream the unsalted butter and granulated sugar until fluffy, then incorporate the eggs one by one, followed by the vanilla extract.

Gradually alternate adding the dry ingredients and the cooled white chocolate mixture to create a smooth cake batter.

Pour the batter evenly into the prepared cake pans and smooth the tops with a spatula. Bake in the preheated oven for about 40 minutes, or until a toothpick inserted in the center comes out clean.

Once baked, remove the pans from the oven and let them cool for 10 minutes. Carefully transfer the cake layers to wire racks and allow them to cool completely before frosting.

In a heatproof bowl, whisk the egg whites and granulated sugar over a double boiler until the mixture is warm to the touch (about 160°F). Remove from heat and whip the mixture until stiff peaks form. Gradually add the room temperature butter, mixing until smooth.

Gently incorporate the instant espresso powder until well combined to create a creamy and glossy Swiss meringue buttercream.

In a small saucepan, heat the heavy cream over medium heat until it begins to simmer. Remove from heat and add the chopped white chocolate, stirring until the chocolate is completely melted.

Allow the ganache to cool and thicken for about 3 hours at room temperature, or until it reaches a pourable consistency for drizzling over the cake.

Once the cake layers and ganache are cooled, place one layer of cake on a serving platter. Spread a generous layer of whipped ganache on top, then add the second cake layer. Repeat with another layer of ganache, and finish with the third cake layer on top.

Apply a thin crumb coat of any remaining ganache around the sides and top, then chill the cake in the refrigerator for about 30 minutes to set.

Frost the outside and top of the cake with the prepared espresso buttercream using an offset spatula. Chill the cake again for about 20-30 minutes to firm up the frosting before proceeding to add the ganache drip.

Gently warm the cooled ganache until it is pourable but not too hot. Drizzle it over the center of the cake's top, allowing it to drip down the sides. Decorate with nonpareils and coffee beans.