Preheat oven to 350°F. Grease and flour three 9-inch round cake pans.

In a stand mixer, beat butter, oil, and sugar until pale and fluffy (about 5 minutes).

Add eggs one at a time, then stir in vanilla.

In a separate bowl, sift flour, baking powder, and salt.

Add flour to the butter mixture in three additions, alternating with sour cream. Begin and end with flour. Mix until just combined.

Divide batter evenly between pans. Bake 25–30 minutes, or until a toothpick comes out with crumbs. Cool in pans 10 minutes, then remove to cool completely.

For the icing, melt butter in a heavy-bottomed pot. Stir in evaporated milk and sugar. Cook over medium-low heat, stirring often, for about 90 minutes.

Icing is ready when thickened, amber-colored, and coats a spoon. Remove from heat, stir in vanilla, and cool slightly (15–20 minutes).

To assemble, place one cake layer on a plate. Spread icing on top, then repeat with remaining layers. Finish by spreading icing around the sides.

Let the cake sit to set the icing. Slice and serve.