Heat oven to 325 F. Spray a muffin tin (regular size, not mini) with nonstick cooking spray or line it with muffin liners.

Add eggs and vanilla extract and beat until evenly incorporated. Add flour and salt and mix on low just until combined. Stir in chocolate chips.

Transfer the cookie dough to the refrigerator while you make the brownie batter (a brief chill in the fridge will cool the cookie dough down and make it easier to handle).

Make the brownie layer: Combine the butter and white sugar in a microwave-safe bowl. Microwave on high until butter is melted, ~1.5 minutes, stirring every 30 seconds. Whisk in the cocoa powder until the batter is smooth.

Set the brownie batter aside for 5 minutes to cool.

When batter has cooled slightly, whisk in salt and vanilla extract. Add eggs, one at a time, beating after each addition. Add flour and stir until smooth.

Place ~1.5 Tbsp of cookie dough into the spaces in the muffin tin. (I like to roll the dough into balls to get them about the same size, but if the dough is too warm to roll into balls, just spoon it in. Dampen your fingers with a bit of water and press the dough to fill the bottom of the muffin tin) Spoon a layer of brownie batter over top, spreading it out so it completely covers the cookie dough.

Bake until the brownie layer is just set, 24 to 26 minutes.

Let the brownies/cookies/cupcokies cool completely before using a knife to gently pop them out of the muffin tin. (They will fall apart if you try to remove them while still warm but should be easy to remove when cool.)