Add the chocolate chips and butter to a medium microwave safe mixing bowl. Heat for 1 minute while stirring in 15 second intervals. If needed, microwave an additional 15 seconds, until the chocolate is melted. Cool slightly.

In a large mixing bowl, use a hand mixer to beat the eggs and egg yolks until fully combined. Mix in powdered sugar and vanilla extract until combined.

Pour in the cooled butter and chocolate mixture until well blended. Then use a spatula to stir in flour, salt and espresso powder, by hand, until just combined.

Divide batter equally between ramekins (about ½ cup if using 6 ounce ramekins) and place them, evenly spaced apart, on a baking sheet.

Bake for 12-14 minutes or until edges are firm but cakes are soft and not quite set in the center.