1) Preheat the oven to 350°F. Lightly grease an 8" square or 9" round pan; or line 12 muffin cups with cupcake papers, and spray the insides of the papers.
2) To make the batter: Beat the soft butter and sugar until thoroughly combined and somewhat lightened in color.
3) Add the eggs one at a time, mixing until just incorporated.
4) Combine the milk and vanilla.
5) Stir one-third of the baking mix into the mixture; stir in half the milk, another third of the baking mix, the remaining milk, then the remaining mix. Scrape the bowl occasionally throughout this process.
6) Spread the batter into the prepared pan. For cupcakes, scoop the batter by level ¼-cupfuls into the prepared muffin tins; a muffin scoop works well here.
7) Bake the cake until a cake tester inserted into the center comes out clean, and the middle springs back when pressed lightly — 30 to 35 minutes for a layer, or 20 to 22 minutes for cupcakes.
7) Remove from the oven. Serve warm; or cool completely, then spread with the frosting of your choice.
Yield: one 8" square layer; or one 9" round layer; or 12 cupcakes.