Preheat oven to 350°F and butter a 13"x 9" cake pan. Place a piece of parchment on the bottom and up the sides of the pan to prevent sticking.
In a large bowl whisk flour, baking powder, and salt to combine.
In another large large bowl of a stand mixer with paddle attachment (or in a bowl using hand held beaters) cream butter and sugar until butter is slightly paler in color and mixture is soft and fluffy.
Add egg and incorporate into creamed mixture until homogenous.
Using a spatula fold dry ingredients into creamed mixture.
On low speed mix milk and vanilla extract into batter until smooth and minimal lumps remain. Do not mix too long.
Pour batter into cake pan and spread into even layer.
Bake for 30 to 40 minutes until cake tester inserted in center comes out clean or with 1 to 2 teeny crumbs. Cool completely, about 1 hour.
In the meantime in a large bowl whip chocolate frosting until thickened and doubled in size. Dollop frosting onto cake and using an offset spatula spread to corners and edges creating swoops and swirls. Sprinkle the top with sprinkles