Preheat oven to 425°F. Spray two (5-ounce) ramekins well with nonstick cooking spray and transfer to a baking sheet. Set aside.

To a small pot, add the butter and semi-sweet chocolate and slowly heat until melted and smooth. Remove from heat and whisk in the powdered sugar until combined. Add the egg, egg yolk, and vanilla and mix until fully combined. Add the flour and espresso powder and mix until just combined, being careful to avoid overmixing the batter. Evenly divide the batter between the two prepared ramekins.

Bake at 425° for 12 to 14 minutes or until the edges are firm and the center is slightly soft. Remove from the oven and let stand for 1 minute. Carefully invert each ramekin onto a small plate. Drizzle with syrup, if desired, and serve.