Step 1

Heat oven to 350 (175C) degrees with rack in the center. Line 2 large rimmed baking sheets with parchment paper.

Step 2

Place the chopped chocolate and butter in a heatproof bowl set over a medium saucepan of simmering water. Stir gently until melted and smooth. Remove from the heat and let cool slightly.

Step 3

Whisk the flour in a small bowl with salt and baking powder.

Step 4

Beat the eggs, sugar, espresso and vanilla in a large bowl using an electric mixer on high speed until light and lofty, 2 or 3 minutes.

Step 5

Drizzle in the melted chocolate on low speed until blended. Add the flour mixture and mix until incorporated. Turn off the mixer and stir in the chocolate chips and nuts.

Step 6

Use a 1/3-cup measure to scoop the dough into 6 mounds on each of the prepared baking sheets, spacing about 2 inches apart. Use a small spatula to scrape the dough out - it's a little gooey.

Step 7

Bake one sheet at a time for 17 minutes. The tops of the cookies should look dry and shiny. Cool the cookies on the pan for 15 minutes, then transfer with a spatula to a wire rack cool completely.