1. Preheat oven to 350°. Coat a mini muffin pan with cooking spray and set aside. Melt chocolate and butter in a small saucepan over low heat, stirring until smooth. Remove from heat, transfer to a medium bowl and add dissolved espresso and vanilla.
2. Add flour to chocolate mixture and stir well. Add egg and stir to incorporate. Add sugars and stir until smooth. Spoon batter evenly into prepared pan. Bake until just cooked through and tops are smooth and slightly puffy, 10–12 minutes.
3. Cool completely (as bites cool, they will collapse and tops will invert) and enjoy.