Preheat oven to 450°F. Spray two 6-ounce ramekins well with nonstick cooking spray and place on a baking sheet. Set aside.
Add the butter and semi-sweet chocolate to a large microwave safe bowl and microwave in 20-30 second increments, making sure to stir well after each increment, until melted and smooth. Remove from the microwave and whisk in the powdered sugar until well combined, then add in the egg, egg yolk, and vanilla and mix until fully combined.
Add in the flour and espresso powder and mix until just combined, making sure not to over mix the batter. Evenly divide the batter between the two prepared ramekins.
Bake at 425° for 12-14 minutes or until the edges are firm and the center is slightly soft. Remove from the oven and allow to stand for 1 minute. Carefully invert each ramekin onto a small plate, serve, and enjoy!