Heat oven to 325°F. Line an 8-inch square pan with parchment overhang.
Beat butter and powdered sugar until light and creamy, 2 to 3 minutes.
Mix in vanilla until smooth.
Whisk flour, cocoa, and salt in a separate bowl.
Add dry ingredients to butter mixture. Mix on low until a soft dough forms.
Fold in mini chocolate chips, if using.
Press dough evenly into the prepared pan. Smooth the top and dock with a fork.
Bake until edges set and the surface looks matte, about 20 to 24 minutes.
Score into bars while warm. Cool completely, then lift out and slice.