Preheat oven to 325 F. Grease a mini muffin tin (you’ll need one that has 24 spaces or will need to bake these in batches; see note if using a regular muffin tin).
Prepare Crust: In a medium mixing bowl, beat together butter (just the portion for the cookie crust) and both sugars until smooth. (Note: you can do this by hand or using a mixer.) Add egg and vanilla and stir to combine. Add flour and salt and mix well. Stir in mini chocolate chips.
Put the chocolate chip cookie dough in the refrigerator to chill while making the brownie filling (chilling the dough briefly will make it easier to work with).
Prepare Brownie Filling: In a small saucepan, heat butter (portion for the filling) just until melted. Remove from heat and whisk in sugar and cocoa powder until smooth. Whisk in egg and 1 Tbsp flour. Set mixture aside until cool enough to touch (this should take about 10 minutes; you can transfer the batter to a mixing bowl and put it in the refrigerator if you want to cool it more quickly.)
Assemble Tarts: Divide the cookie dough evenly between the spaces of a muffin tin. (Just drop the dough into the spaces to start, and rearrange as needed utnil each space contains about the same amount.) Roll each ball of dough into an even round. Use your fingers to press the dough out into the sides of the muffin tin, creating a thin tart shell. (Note: if the dough sticks, dip your fingertips in water first.)
Divide brownie batter evenly between tart shells.
Bake until the brownie filling is puffed and just barely set in the middle, 9 to 11 minutes.
If you'd like, add a tiny pinch of flaked sea salt (like Maldon) to the top of each tart as soon as they come out of the oven. (This is optional, but I always love the balance and texture that comes from a tiny pinch of salt on sweet baked goods.)
Let tarts cool in the muffin pan.
Store in a sealed container at room temperature for up to 3 days.