Step 1: Preheat your oven to 350°F (175°C) and line an 8×8 inch square baking pan with parchment paper, leaving some overhang on the sides.
Step 2: In a large bowl, whisk together the melted butter and sugar until well combined and slightly glossy.
Step 3: Add the eggs one at a time, whisking thoroughly after each addition. Stir in the vanilla extract.
Step 4: Sift the cocoa powder, flour, salt, and espresso powder over the wet ingredients. Fold gently with a spatula until just combined. Do not overmix.
Step 5: Fold in the dark chocolate chips and pour the batter into the prepared baking pan, smoothing the top with a spatula.
Step 6: Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs attached.
Step 7: Allow the brownies to cool completely in the pan before lifting them out using the parchment paper handles.
Step 8: Cut into 16 squares. To decorate, press two chocolate wafer cookies into the sides of each brownie at an angle to look like wings, and add edible eyes if desired.