Preheat the oven to 350°F. Lightly spray 2 dozen mini muffin cups with cooking spray. Depending on how full you fill them with cookie dough, you may need to spray a few more. Set aside.
Brown the butter. In a small saucepan over medium heat, melt the butter. Once it has melted, whisk constantly while browning. It will melt, then get foamy, then the foam will subside, and brown bits will start forming on the bottom.
Once you see the brown bits, remove the pan from the heat. Continue whisking for another minute, then pour the butter into a small bowl so it doesn’t continue to heat in the saucepan. Make sure to get all the brown bits out of the saucepan. Let cool slightly.
In a large bowl, or the bowl of a stand mixer, whisk together the egg, brown sugar, granulated sugar, peanut butter and vanilla until combined.
Add the cooled browned butter and mix until smooth.
Add the flour, baking soda and salt and mix until just combined. Don’t over-mix.
Stir in the chocolate chips.
Fill the mini muffin cups with cookie dough, filling 2/3 to completely full, depending on how big you would like them.
Bake for 6-8 minutes, or until the tops are set and a toothpick inserted in the center of one comes out clean, or just with a few moist crumbs.
Allow the cookies to cool in the pans for at least 15 minutes before removing them or they will break.
Store in an airtight container at room temperature for up to 7 days.