Step 1

Preheat the oven to 400 degrees F.

Step 2

Brush four 6-ounce ramekins with the melted butter and sprinkle lightly with sugar. Place the ramekins on a baking sheet.

Step 3

Combine the chocolate and butter in a medium microwave-safe bowl. Microwave on high in 30 second intervals, stirring in between each with a rubber spatula until completely melted and smooth, about 1 1/2 minutes total. Add the vanilla, espresso powder and salt into the chocolate mixture and stir to combine.

Step 4

Add the eggs, egg yolks and 1/4 cup sugar to the bowl of a stand mixer fitted with a whisk attachment. Whisk on medium-high until fluffy and pale, about 1 1/2 to 2 minutes.

Step 5

With the mixer on low, add the chocolate mixture to the egg mixture, mixing until just combined. Remove the bowl from the stand mixer and gently fold in the flour using a rubber spatula.

Step 6

Divide the batter equally among the 4 prepared ramekins. Bake until the sides are firm but the centers are still soft, 12 to 14 minutes. (A cake tester inserted into the center of each cake should not come out clean.) Do not overbake.

Step 7

Let the cakes cool for 1 minute. Invert onto serving plates and serve warm.