Preheat the oven to 425 degrees. Line a muffin tin with cupcake liners or grease the cups well.

In a large mixing bowl, whisk together the cocoa, flour, brown sugar, espresso powder, baking powder, soda, and salt until no lumps remain. Stir in baking morsels.

In another bowl, combine the eggs, milk, vanilla, and vinegar. Whisk together until smooth.

Add the egg mixture and melted butter the dry mixture and stir with a wooden spoon until no white remains.

Scoop the mixture into 12 muffin cups. It will fill each cup almost to the top. Bake for 16-20 minutes or until a toothpick inserted in the center of the muffin comes out clean. Remove from the oven and allow to cool in the pan for 10 minutes before removing. If desired, before serving drizzle each muffin with caramel sauce and sprinkle with sea salt.

These muffins can be served warm or at room temperature. They'll keep in an airtight container for up to 5 days.