Preheat the oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a large mixing bowl, sift together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Add the eggs, milk, vegetable oil, and vanilla extract to the dry ingredients. Beat on medium speed for 2 minutes until well combined.
Carefully stir in the boiling water. The batter will be thin.
Divide the batter evenly between the prepared cake pans.
Bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cakes from the oven and let them cool in the pans for 10 minutes. Then, turn them out onto wire racks to cool completely.
To prepare the dark chocolate ganache, place the chopped semi-sweet chocolate in a heatproof bowl.
Heat the heavy cream in a small saucepan over medium heat until it just begins to simmer. Pour the hot cream over the chopped chocolate and let it sit for 2 minutes.
Add the softened butter and stir gently until the mixture is smooth and glossy. Allow the ganache to cool slightly until it thickens to a spreadable consistency.
For the white chocolate decorations, melt the white chocolate in a microwave-safe bowl in 20-second intervals, stirring between each, until smooth.
Line a baking sheet with parchment paper. Using a piping bag or a small spoon, create geometric shapes (such as triangles and diamonds) with the melted white chocolate on the parchment paper. Refrigerate until firm.
Once the cakes are completely cool, spread a thin layer of ganache on top of one cake layer. Place the second cake layer on top.
Cover the entire cake with the remaining ganache, smoothing it evenly over the top and sides.
Carefully arrange the chilled white chocolate geometric shapes on the cake surface to create a unique design pattern.
Refrigerate the cake for at least 30 minutes to set the ganache before slicing and serving.