Preheat oven to 350° and line a 9x13 inch pan with parchment paper, then spray with oil or just spray pan with oil.
Melt butter in medium saucepan, remove from heat, whisk in sugar, eggs, vanilla, cocoa powder, flour, salt and baking powder, stirring after each addition. Gently stir in chocolate chips.
Pour into prepared pan and bake for 25-30 minutes. DO NOT OVERBAKE. Brownies will settle as they cool. Edges should just start pulling away from sides of pan and the center should no longer be glossy. Cool completely in pan on cooling rack.
If frosting, frosting may be poured over hot brownies or cooled, the frosting spreads a bit easier over warm brownies, cool completely before cutting.
Once cooled, I find it easiest to freeze (up to several weeks if properly wrapped). When ready to cut, remove from freezer, let sit 20 minutes then cut brownies, they are a bit easier to handle when slightly frozen.
Melt butter in small saucepan. Stir in cocoa powder, then remove from heat. Whisk in powdered sugar, milk, vanilla and pinch of salt.
Once smooth, pour over warm or cooled brownies and let cool completely. Cut into 2" squares.
I find it easier to refrigerate or freeze for 20-30 minutes before cutting so the frosting cuts smoothly. Store in airtight container in refrigerator for 1 week or freeze individual squares for lunch box or quick snacks.
Store on counter 2-3 days in airtight container or fridge up to one week. Wrap well and freeze up to 3 months. (pssst, they'll never last that long).