Brownie Filling:
Microwave the butter, sugar, and cocoa powder for 30 seconds. Stir, then microwave for 10 more seconds. Stir until smooth. Mix in the salt, milk, and vanilla. Whisk in the egg until combined, then fold in the flour just until no dry streaks remain. Set aside.
Chocolate Chip Cookie Dough:
Preheat the oven to 350°F and lightly grease six 3-inch tart pans. Stir together the melted butter, brown sugar, and granulated sugar until smooth. Mix in the egg yolk, milk, and vanilla. Add the flour, baking soda, and salt, then stir just until combined. Fold in the chocolate chips.
Assemble & Bake:
Divide the cookie dough evenly among the tart pans and press into a thin, even layer over the bottom and halfway up the sides. Spoon the brownie batter into the center and gently spread into an even layer, leaving a small cookie border around the edge. Bake for 18–20 minutes, or until the cookie edges are golden brown and the brownie center is just set. Cool in the pans for 15 minutes before removing. Sprinkle with flaky sea salt, if desired, and serve warm or at room temperature.
