I found this recipe in 2010. Have made it many times since and have updated a few things. This is the original recipe, but I periodically change the espresso powder for decaf and the dark chocolate for chips. It is delicious and very rich.
3/4 cup all-purpose flour (3.4 ounces)
2/3 cup unsweetened cocoa
5 tsp instant espresso powder
1 1/2 tsp baking powder
1/4 tsp salt
1/4 cup butter, softened
2/3 cup granulated sugar
2/3 cup packed brown sugar
4 eggs
1 1/2 tsp vanilla extract
2.6 ounce dark chocolate
2 tbsp powdered sugar
Weigh or lightly spook flour into dry measuring cups. Level with a knife and stir together with the cocoa, espresso, baking powder and salt.
Place butter in a large mixing bowl and beat well for 1 minute. Add in the sugars (both white and brown) and beat well for 5 minutes. Add in the eggs and vanilla and beat. Fold flour mixture into the sugar mixture. Fold in the chocolate. Divide the batter evenly between 10 (4oz) ramekins. Arrange the ramekins on a jelly roll pan and refrigerate for 4 hours. This can stay in the fridge for up to 2 days.
Preheat the oven to 350oF.
Let ramekins stand at room temperature for 10 minutes before baking. Uncover and bake for 21 minutes or until cakes are puffy and slightly crusty on top. Sprinkle evenly with powdered sugar and serve immediately.