Combine the melted butter and canola oil in a large bowl and stir well. Add sugar and stir again to combine.

Add egg, egg white, and vanilla extract. Mix thoroughly until all ingredients are well-combined, then stir in buttermilk.

In a separate bowl, whisk together the flour, cornstarch, baking powder, and salt.

Using a spatula, gently fold the dry ingredients into the wet, taking care not to over-mix. Once the dry ingredients have been about half-way stirred into the wet, stir in add-ins (if using) and continue to fold in to batter until just combined.

Cover bowl with a dry towel and allow batter to rest for 15-60 minutes (see note).

Preheat oven to 425F (215C) and line a 12-count muffin tin with paper liners.

Once oven has preheated, portion batter into prepared tin, filling each liner at least 3/4 of the way full.

If desired, sprinkle tops generously with sugar and transfer muffin tin to preheated oven.

Bake on 425F (215C) for 8 minutes and then, without opening the oven door, reduce the oven temperature to 350F (175C) and bake another 12 minutes, or until tops are just beginning to turn golden brown.

Allow to cool and enjoy!