Preheat oven to 325°F. Grease an 8×8-in. square baking pan and line the bottom with parchment paper.

Mix the cake batter by creaming butter, oil, and sugar until light and fluffy. Add eggs and egg whites, then gradually add dry ingredients alternately with buttermilk. Lastly, mix in the vanilla extract.

Transfer the batter to the prepared baking dish and bake for 30-40 minutes. Let the cake cool completely in the pan on a wire rack, then transfer to the freezer for at least an hour.

Prepare the vanilla buttercream by beating butter until creamy, then adding confectioners’ sugar, heavy cream, vanilla extract, and salt. Transfer to a pastry bag and refrigerate.

Make the chocolate fudge frosting by combining butter, granulated sugar, buttermilk, dark corn syrup, cocoa, and salt in a saucepan. Bring to a boil, then whisk in confectioners’ sugar until smooth. Remove from heat and stir in vanilla. Let cool until just above room temperature.

Decorate the cake by piping ribbons of buttercream over the cake, then pour the fudge icing over the cake, covering the buttercream “bumps” completely. Chill to set before slicing and serving.