Have all ingredients at room temperature.

Preheat oven to 350°F with a rack in the center. Butter and flour an 8 x 8-inch baking pan, tapping out any excess flour.

Into a medium mixing bowl, sift together the flour, baking powder, and salt.

In a large mixing bowl or the bowl of a stand mixer fitted with the paddle, beat sugar, butter, and safflower oil together, starting on low speed and increasing to medium when incorporated, for 90 seconds.

Beat in eggs, yolks, and vanilla until incorporated.

Reduce mixer speed to low. Alternate between adding the flour mixture (in 3 parts) and the buttermilk (in two parts), starting and ending with flour. Mix just until batter is smooth — do not overmix.

Spread batter evenly into prepared pan.

Bake for 28-32 minutes. When baked, cake will be set and slightly springy in the center and will have begun to pull away from the sides of the pan. A cake tester inserted in the center should come out with a few crumbs clinging to it, but not liquid batter.

Set pan over a wire rack to cool completely before frosting. (You may leave the cake in the pan to frost it or gently remove it when cool enough to handle.)

Melt the chocolate chips in a double boiler or in short bursts (about 30 seconds each) in the microwave, stirring between bursts.

When melted, stir in sour cream, milk, vanilla and salt until smooth and glossy.

Spread immediately onto cooled cake.

Decorate with sprinkles. Frosting will set over time.