Preheat and prep: Set your oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone mats. This helps the cookies bake evenly and release cleanly.

Whisk the dry ingredients: In a medium bowl, whisk flour, cocoa powder, baking soda, and salt. If using espresso powder, whisk it in now. Break up any cocoa lumps so the dough stays smooth.

Cream butter and sugar: In a large bowl, beat the butter and sugar with a hand mixer or stand mixer until light and fluffy, about 2 minutes. This step adds a touch of air, which helps with the crisp texture.

Add egg and vanilla: Beat in the egg and vanilla until combined. Scrape down the bowl so everything mixes evenly.

Combine wet and dry: Add the dry ingredients to the butter mixture. Mix on low just until the dough starts coming together. If it looks dry or crumbly, add the milk, 1 tablespoon at a time, until the dough is soft but not sticky.

Chill briefly: Form the dough into a flat disk, wrap it, and chill for 20–30 minutes. Chilling helps the dough roll thin and keeps the cookies from puffing too much.

Roll it thin: Lightly flour your surface and rolling pin. Roll the dough to about 1/8 inch (3 mm) thick for crisp wafers. Rotate and lift the dough occasionally to prevent sticking. Dust lightly with flour as needed.

Cut shapes: Use a round cutter (2 to 2.5 inches works well) or a fluted cutter for a classic wafer look. Re-roll scraps gently; avoid overworking the dough to keep the texture snappy.

Arrange and prick: Place cookies 1 inch apart on prepared sheets. For extra-flat wafers, lightly prick each cookie once or twice with a fork. Pricking helps steam escape and keeps them level.

Bake: Bake 8–10 minutes, rotating pans halfway. The edges should look dry and set, and the centers should lose their sheen. They’ll firm up more as they cool.

Cool completely: Let cookies rest on the sheet for 2 minutes, then transfer to a rack. Cooling fully is key to maximum crispness.