Preheat oven to 375 degrees.
Line 24 muffin/cupcake tins with paper liners (or grease and flour unlined tins). In a small saucepan, melt the butter. Cool it slightly. While butter is cooling, in a large bowl, combine the flour, brown sugar, salt and baking powder using a fork to mix and break up sugar. Add chocolate chips, stirring in.
After butter is cooled, stir in milk, eggs and vanilla with a wire whisk. Stir this mixture into the flour mixture JUST UNTIL MOISTENED. Do not over mix (makes tough muffins). Batter will be on the thick side and lumpy. Divide into prepared cups (about 3/4 full).
Bake 15-20 minutes or until tooth pick comes out clean. Cool on wire rack.