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Combine butter, oil, and sugar in a large bowl and whisk together.
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Add egg, egg white, and vanilla extract. Whisk together thoroughly, until all ingredients are well-combined, then stir in buttermilk until combined.
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In a separate bowl, whisk together flour, baking powder, corn starch, and salt.
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Fold dry ingredients into wet gently, taking care not to overmix. Once about half of the dry ingredients have been incorporated into the batter, add in chocolate chips and continue to fold in to batter until all ingredients are just combined.
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Cover bowl with a dry towel and allow batter to sit while your oven preheats (see note).
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Preheat oven to 425F (215C) and line a 6-count jumbo muffin tin with paper liners.
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Once your oven has preheated (maybe give it a few extra minutes or check your oven thermometer, my oven often says it's reached temperature when it really hasn't yet!), portion batter into prepared muffin tin, filling each tin all the way to the top with batter (without overflowing).
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Sprinkle tops generously with additional sugar and place muffin tin in preheated oven.
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Bake on 425F (218C) for 8 minutes and then, without opening the oven door, reduce the oven temperature to 350F (175C) and bake another 15 minutes, or until tops are just beginning to turn golden brown
Note: if you are using a standard 12-count muffin tin rather than jumbo-sized, they will not need to bake as long, check your muffins after 7-8 minutes on 350F (177C)).
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Allow muffins to cool in the muffin tin for 5-10 minutes then carefully remove from muffin tin and enjoy!