Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a bowl, whisk flour, cocoa, baking powder, salt, and espresso powder.
In another large bowl, beat butter and sugar until fluffy. Mix in eggs and vanilla.
Gradually stir in dry ingredients until just combined. Fold in chocolate chips and nuts.
Divide dough in half. Shape each into a 10x3-inch log on the prepared sheet.
Bake for 25-30 minutes, until firm and cracked on top. Let cool 10 minutes.
Lower oven temp to 325°F. Slice logs diagonally into 3/4-inch slices.
Place slices cut side down. Bake 10-12 minutes, flip, then bake another 10-12 minutes until dry and crisp.
Cool completely on a wire rack before storing.