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Preheat the oven to 350F. Line a muffin tray with 9-12 liners, depending on size of muffins desired.
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Whisk together flour, sugar, cocoa powder, baking powder, baking soda, salt, and espresso powder, if using.
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In another bowl, whisk together the melted butter, eggs, buttermilk and vanilla extract.
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Pour the wet mixture into flour mixture and gently stir to combine.
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Stir in most of chocolate chips, while setting some on the side for decorating the tops.
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Spoon the batter into each of the liners to the very top for larger muffins or 3/4 for smaller muffins.
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Sprinkle with the remaining chocolate chips and the sparkling sugar, if using.
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Bake for 16-20 minutes, or until a toothpick inserted into the centers comes out clean. Remove from the oven and let cool before removing from the tin.