• Preheat the oven to 350°F. Grease a 9x13-inch baking pan and line it with parchment paper, leaving some overhang on the sides so the brookies are easy to remove after baking.

Make the Brownie:

• In a saucepan over medium-low heat or the microwave, melt the chopped semi-sweet chocolate and butter together until smooth. Remove from the heat and stir in the instant espresso powder. Let the mixture cool slightly.
• In a large mixing bowl, beat the eggs and granulated sugar for 5–7 minutes, or until the mixture is pale, thick, and slightly increased in volume.
• Add the vanilla extract and the slightly cooled chocolate-butter mixture. Mix until smooth and combined.
• Add in the flour, cocoa powder, and salt. Gently fold until just combined. Fold in the chocolate chips. Set aside.

Make the Cookie Dough:

• In a separate large bowl, beat the softened butter, brown sugar, and granulated sugar together until light and creamy, about 2–3 minutes.
• Add the egg and vanilla. Mix until combined.
• Add the flour, baking soda, and kosher salt. Mix until a cookie dough forms. Fold in the semi-sweet chocolate chips.

Assemble

• Spread about ½ of the brownie batter into the prepared pan.
• Take about ⅓ of the cookie dough and flatten it into small, rough pieces. Place the cookie dough pieces over the brownie batter. They do not need to cover the brownie completely.
• Dollop the remaining brownie batter over the cookie dough layer and gently spread it out. It is okay if some cookie dough still peeks through.
• Flatten the remaining ⅔ of the cookie dough into small rough pieces and arrange them over the top. Leave some brownie gaps showing, but make sure most of the top has visible cookie dough. This helps the brookies bake with a more golden, cookie-like top. Top with additional chocolate chips if desired.
• Bake for 40 to 45 minutes, or until the cookie dough on top is golden, the edges are set, and the center no longer jiggles. A toothpick should come out mostly clean (no raw wet brownie batter).
• Let the brookies cool completely in the pan, then lift the parchment paper and cut into bars. Top with flaky sea salt if desired.