Preheat your oven to 350°F (175°C) and line your baking sheets with parchment paper.

Melt semisweet chocolate chips and unsalted butter in a microwave-safe bowl, heating in 30-second intervals until smooth.

In a large bowl, beat eggs, granulated sugar, and vanilla extract with an electric mixer for 2-3 minutes until thick and creamy.

Gently mix the cooled chocolate and butter mixture into the egg mixture until fully combined.

Whisk together flour, baking powder, and salt in a separate bowl. Fold this dry mixture into the wet mixture.

Drop rounded spoonfuls of the brownie batter onto the prepared baking sheets, spacing them about 2 inches apart.

Bake for 8-10 minutes until the tops are set and slightly cracked. Cool on wire racks.

For the filling, beat unsalted butter with light brown and granulated sugars until fluffy, then mix in milk and vanilla.

Add flour and a pinch of salt to the butter mixture, mixing until just combined. Fold in mini chocolate chips.

Assemble the sandwiches by spreading the cookie dough filling on half of the cooled brownie cookies and topping with the remaining cookies.

Store in an airtight container at room temperature for up to 3 days or refrigerate for up to 1 week.