Spray a 9-inch round cake pan with non-stick cooking spray. Cover with a parchment circle. Spray again. Flour the bottom and sides of the pan and knock out the excess.
In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
In the bowl of an electric mixer, using the paddle attachment, cream the butter and sugar until light. Mixture will not look fluffy.
Add the egg and blend well. On low speed, add 1/3 of the dry ingredients, blending just until combined.
Add the vanilla extract to the milk, and add ½ the milk into the mixer. Blend until combined. Add another 1/3 of the dry ingredients, followed by the remaining milk mixture. Add the final 1/3 of the dry ingredients and blend well.
Pour batter in the prepared pan, and bake for approximately 35 minutes, or until a small sharp knife inserted in the center comes out clean.
Cool cake until just barely warm, and remove from pan. Place cake onto a cardboard cake circle or platter.
Divide cake in half, reserving the top half. Spread bottom half with Vanilla Pastry Cream and slide reserved half of cake on top.
Pour ganache over the top of the cake.
Fill a parchment cone or pastry bag with melted white chocolate, if desired. Cut a small opening in the bottom, and drizzle the chocolate over the top of the cake in concentric circles. Using a toothpick or wooden skewer, pull chocolate through ganache from the center out to make decorative lines. Repeat around cake.