Preheat the oven to 350°F. Melt the brown butter and brown sugar together in the microwave for about 30-60 seconds until the butter is fully melted.
Whisk the melted butter and sugar together well until very smooth and glossy. Heat the mixture for an additional few seconds if it seems to be cooling down too quickly. You want it to be fairly warm and viscous.
Whisk the cocoa powder into the butter and sugar mixture. It will look very lumpy and that’s okay! Add in the egg, vanilla and apple cider vinegar and continue whisking until smooth.
Switch to a spatula and gently fold in the flour and salt until fully incorporated. Then mix in the chocolate chips.
Lightly grease a mini muffin pan with cooking spray. Spoon the brownie batter evenly into the muffin cups, filling up each cup almost to the top.
Transfer to the oven and bake for 12-14 minutes. The brownies are done when the tops are set and a toothpick comes out with fudgey crumbs attached.
Let the brownies cool in the pan for about 15-20 minutes before removing and enjoying! Finish with a sprinkle of flaky salt if desired.
