Preheat oven to 350 degrees F. Grease and line a 9 x 13-inch baking pan with parchment paper.
Add flour, baking powder, and salt to the bowl of a stand mixer fitted with the paddle attachment. Add softened butter and mix on low speed for 2 minutes. Using a rubber spatula, scrape down the sides of the bowl and mix on low speed for another 2 minutes.
Add the sugar ½ cup of milk. Mix on low speed for 4 minutes.
In a large glass measuring cup, combine remaining ½ cup of milk, vanilla extract, and egg whites. Add to the batter in 3 parts, mixing on low speed after each addition. Once all of the liquids have been added, continue mixing on low speed for a total of 5 minutes.
Fold in chocolate chips with a rubber spatula.
Pour the batter into the prepared baking pan and bake for 35-40 minutes, or until a wooden skewer inserted into the center comes out clean.
Remove cake from oven and let cool 15 minutes in the pan. Using the parchment paper, lift the cake out of the pan and transfer it onto a wire rack to cool completely.
To make the buttercream, beat the butter until creamy. Add the confectioner’s sugar 1 cup at a time. Add heavy cream and beat until light and fluffy.
Spread the buttercream over the cooled cake and top with mini chocolate chips. Cut into squares and serve!