Preheat the oven to 350°F. Grease three 8-inch round cake pans with cooking spray.
In a large bowl, whisk together the flour, cocoa powder, baking soda, baking powder and salt.
In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and sugar until light and fluffy. Add the eggs one at a time, beating between each addition. Add the sour cream and vanilla, beating until combined.
Alternately add the milk and flour mixture in three increments just until combined. Fold in the mini chocolate chips.
Divide the batter equally among the three cake pans.
Bake the cakes until a toothpick inserted comes out clean, 22 to 25 minutes. Let the cakes cool for 10 minutes in the pans then transfer them to a rack to cool completely while you make the frosting.