Maldon or other large salt, for topping (optional)
Preheat the oven to 325 F. Grease a mini muffin tin.
To make the cookie crust, in a small bowl beat together the butter and sugar until light and fluffy (do this by hand with a wooden spoon or in a standing mixer). Add the egg yolk and mix. Add the flour and salt and mix until well combined. Stir in mini chocolate chips. Put the bowl in the refrigerator to chill while making the brownie filling (this will make it easier to handle).
For the brownie filling, heat the butter on the stove or in the microwave, just until melted. Whisk in sugar and cocoa powder until smooth. Set aside until only slightly warm to the touch, about 5 minutes. Using a wooden spoon stir in the egg and vanilla until evenly incorporated. Add flour and stir until smooth.
Divide the cookie dough into 12 even portions (about 2 teaspoons each) and press them into the prepared muffin tin to form tart shells. Pour the brownie filling into each of the shells.
Bake for 9-10 minutes, just until the brownie starts to set around the edges (the centers will still be gooey). Be careful not to overtake these – pull them from the oven as soon as you see that the brownie edges are setting.
Let cool at room temperature in the muffin tin (Note: be sure to let these cool completely or they will crumble – patience is key!). Sprinkle with salt (optional).