I make these bars all the time because they only require one large mixing bowl and are so easy and delicious. You can use any combination of chips, I like chocolate chips, butterscotch chips and dark chocolate chips. Of course I add a batch of buttercream frosting on the top after they have cooled!
11 tbsp (156g) butter
2 cups + 2 tbsp (454g) firmly packed brown sugar
1 tsp salt
2 tsp (9g) vanilla extract
3 large eggs
2 1/4 tsp (9g) baking powder
3/4 cups (326g) unbleached all-purpose flour
4 cups (680g) chocolate chips, or a combination of different flavored chips
Preheat the oven to 350°F. Line a 9" x 13" pan with parchment paper.
In large bowl, melt butter and stir in brown sugar. Add salt and vanilla, stirring until combined. Allow the mixture to cool slightly, then add eggs one at a time, mixing well after each addition. Add the baking powder and flour, stirring to combine. Stir in the chips or other additions of your choice.
Scoop batter into the prepared pan, spreading it to the edges with a wet spatula (or your wet fingers tips).
Bake bars for 30 to 32 minutes, until risen, and top is shiny and golden. A cake tester inserted in the center won't come out clean, the center of bars will look undercooked, just as long as the edges of bars look done. As they cool, the center will solidify.
Remove the bars from the oven, and use a heatproof spatula or fingertips to press down the risen edges; this will make bars flat instead of humped in the middle.
Let bars cool completely before cutting. Once bars are cool, lift parchment paper and bars to cutting board to cut and serve.
(Adapted from King Arthur Flour)