Cover leftover cake tightly and store in the refrigerator for 5 days.
Make ahead tip: Prepare cake through step 5. Cover the cake tightly and refrigerate for up to 2 days or freeze up to 3 months. Bring to room temperature, make the frosting, frost, and serve. Frosted cake freezes well, up to 3 months. Thaw overnight in the refrigerator, bring to room temperature or serve cold.
If you don't have buttermilk, you can add 1 Tablespoon fresh lemon juice or white vinegar to a large liquid measuring cup. Then add enough regular room temperature milk (whole milk is recommended) to make 1 1/2 cups total. Stir and let sit for 5 minutes. This soured milk can be used in the recipe instead of buttermilk.
For cupcakes: For about 2 dozen cupcakes, fill the cupcake liners halfway and bake for about 20-22 minutes. Same oven temperature.
This recipe was adapted from Sally's Baking Addiction recipe: Best Banana Cake I've Ever Had