Preheat oven to 350ºF. Grease a 9X13" pan and set aside.
In a food processor place zucchini chunks, melted butter, eggs, and vanilla. Process until smooth. If you are making brownies without a food processor or in a blender, see RECIPE NOTES.
Add sugar, cocoa powder, baking powder, salt, and flour. Process until just combined. Add chocolate chips and pulse a few times to break chips up just a bit.
Pour into prepared 9X13" pan and bake for 40-44 minutes, or until toothpick inserted in middle comes out without wet batter. Cool completely in pan while making peanut butter frosting.
Peanut Butter Frosting:
In a medium mixing bowl, use a hand mixer to beat together on LOW SPEED peanut butter, softened butter, powdered sugar, vanilla, and salt. Once all the ingredients are incorporated, scrape down bowl. Increase speed to HIGH and beat until smooth and fluffy, about 1 minute.
Add 1 tablespoon of milk and mix well. If frosting is too thick, add a splash more of milk until you reach desired consistency. Spread peanut butter frosting on cooled brownies.
In a microwave safe bowl place chocolate chips and milk. Melt in microwave for 30 seconds. Chips will not be fully melted, but stir until smooth.
Pour glaze over peanut butter frosting and spread with an offset spatula. Chill brownies for 15-30 minutes to set chocolate glaze before cutting into bars.
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Tips and Tricks for Making Zucchini Brownies
Don't have a food processor? No problem. Use a grater and measure out 3 cups of shredded zucchini instead.
If you don't have a kitchen scale, cut enough cubed zucchini to fill 4 cups.
This recipe can easily be halved and baked in a 8" or 9" square pan. Bake for 25-30 minutes.