Step 1

Arrange 2 racks to divide the oven into thirds and heat the oven to 375ºF. Line 2 baking sheets with parchment paper.

Step 2

Place 1/4 cup all-purpose flour, 1/4 teaspoon baking powder, and 1/8 teaspoon kosher salt in a small bowl and whisk to combine. Coarsely chop 5 ounces dark chocolate (about 1 1/4 cups) and 2 ounces unsweetened chocolate (scant 1/2 cup).

Step 3

Prepare a double boiler or bring a medium saucepan filled with an inch or two of water to a simmer over medium heat. Add 2 tablespoons unsalted butter and the dark and unsweetened chocolates to the double boiler or place in a metal bowl that fits over the saucepan without touching the water. Heat, stirring often, until the butter and chocolate are melted. Remove from the heat and stir until smooth. (Alternatively, melt the butter and chocolates in the microwave in 30-second bursts, stirring between each burst.)

Step 4

Place 2 large eggs in the bowl of a stand mixer (or large bowl if using an electric hand mixer). Beat with the whisk attachment briefly on medium speed to break them up. Add 2/3 cup granulated sugar, 1 1/2 teaspoons brewed espresso, and 1 teaspoon vanilla extract and beat on high speed until thickened, satiny, and lightened in color, about 15 minutes.

Step 5

Add the chocolate mixture into the egg mixture and fold until partially combined (there should still be some streaks). Add the flour mixture and carefully fold it in until just combined. Add 3/4 cup mini chocolate chips and fold to combine. If the batter is very runny, let it rest until it thickens slightly, about 5 minutes.

Step 6

Drop the batter by heaping teaspoonfuls onto the baking sheets, spacing them at least 1 1/2-inches apart, 30 per sheet.

Step 7

Bake for 5 minutes. Rotate the baking sheets between racks and from front to back. Bake until puffed and cracked, 2 to 3 minutes more. Let cool completely on the baking sheets.