Position a rack in the center of the oven and preheat to 350°F. Lightly coat two 8-inch round cake pans with baking spray (or butter and flour). For extra insurance, line the bottoms with parchment paper rounds.

In a medium bowl, whisk together the flour, baking powder, and salt. Set aside 2 to 3 tablespoons of this mixture in a small bowl; you will toss the chocolate chips with it later.

In a large bowl, beat the softened butter and sugar together until light and fluffy, about 2 minutes. Beat in the eggs one at a time, then add the oil, milk, and vanilla; mix until well combined.

Add the dry ingredients (minus the reserved 2 to 3 tablespoons) to the wet mixture and stir until just combined. Toss the mini chocolate chips with the reserved flour mixture, then fold them into the batter. The batter will be thick but spreadable.

Divide the batter evenly between the prepared pans and smooth the tops. Bake for 32 to 35 minutes, or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes, then turn them out onto wire racks to cool completely.

If not frosting immediately, wrap the cooled cake layers tightly in plastic wrap and store at room temperature for up to 3 days, or refrigerate for up to 3 days. For longer storage, wrap well and freeze for up to 1 month; thaw before frosting.

In a large bowl with a hand mixer or in a stand mixer fitted with the paddle attachment, beat the softened butter on medium-high speed until smooth and creamy, about 2 to 3 minutes.

Reduce speed to low and gradually add the sifted powdered sugar, one cup at a time, mixing until incorporated. Add the vanilla and heavy cream, then beat on medium speed until light and fluffy, about 1 minute.

If using the mini chocolate chips in the frosting, fold them in with a spatula just before frosting the cake. (If making the frosting ahead, wait to add the chips until you are ready to use.)

Place one cooled cake layer on a serving plate, spread a generous amount of frosting on top, then add the second layer. Frost the top and sides of the cake with the remaining buttercream. Slice and serve.