Melt white chocolate with whole milk in a microwave-safe bowl until smooth, stirring every 30 seconds to prevent burning. Add instant espresso powder, mix well, and set aside to cool slightly. Preheat your oven to 350°F (175°C) and grease three 6-inch cake pans with butter.

In a separate bowl, whisk together all-purpose flour, baking powder, and salt.

In a large mixing bowl, cream together room temperature unsalted butter and granulated sugar using a stand mixer on medium speed for about 3-5 minutes until light and fluffy. Add eggs one at a time, mixing well after each addition, followed by vanilla extract.

Gradually incorporate the cooled white chocolate mixture into the butter-sugar mixture. Then alternate adding the flour mixture and milk mixture to the batter, starting and ending with the flour.

Divide the batter evenly among prepared cake pans and bake for approximately 35-40 minutes or until a toothpick inserted comes out clean.

Remove the cake layers from the oven and allow them to cool in the pans for 10 minutes, then transfer to wire racks to cool completely.

Prepare the espresso Swiss meringue buttercream by whisking egg whites and sugar over a double boiler until the mixture reaches about 160°F (70°C) and the sugar has dissolved.

Transfer to a stand mixer and whip until stiff peaks form. Gradually add unsalted butter while mixing, then add instant espresso powder until smooth.

For ganache, combine chopped white chocolate and heavy cream in a microwave-safe bowl. Microwave in 30-second intervals until melted.

Assemble the cake by placing one cake layer on a serving plate, spread ganache on top, and repeat with remaining layers. Ice the exterior with espresso buttercream.

Apply a thin crumb coat over the cake and chill in the refrigerator for about 30 minutes. Drizzle cooled ganache over the top.

Decorate the cake with optional remaining espresso buttercream and decorative items like coffee beans.