In a large mixing bowl, combine butter, oil and sugar.
Add eggs, vanilla and buttermilk until combined on low or by hand.
In a bowl, combine flour, baking powder, corn starch and salt.
Fill 3/4 full and sprinkle with sugar.
How to store muffins:
allow them to cool in the tin for a few minutes and transfer to a wire rack
store in an airtight container or ziplock bag on a paper towel to absorb any excess moisture
to freeze, wrap muffins in saran wrap and place them in a freezer bag to store up to 3 months