Mix dry stuff: Turn oven to 350°F. Whisk 1¾ cups flour, 1 tbsp cornstarch, 1 tsp baking powder, ½ tsp baking soda, 1 tsp salt.
Beat wet base: Beat ½ cup softened butter, 1 cup sugar, 2 tbsp oil, 1 tsp vanilla, 2 tsp maple extract for 3-4 minutes till fluffy.
Add eggs: Beat in 2 eggs and 1 yolk one at a time, mixing well after each and scraping the bowl.
Alternate ingredients: Mix ½ cup buttermilk with ½ cup maple syrup. On low speed, add dry mix in three parts, switching with buttermilk mixture.
Fill and bake: Line tin with 12 liners, fill each ⅔ full. Bake 18-20 minutes till a toothpick comes out clean.
Cool down: Let sit in tin 10 minutes, move to wire rack, cool completely before frosting.