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Beat together sugar and oil, then add in eggs, cocoa powder, flour, and baking soda and beat again until thoroughly mixed.
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Line a square 9x9 inch baking tray with parchment, and spray with nonstick. Pour in brownie mix and bake at 350 degrees for 20-25 minutes.
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Remove from oven and set aside to cool.
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In a new bowl, beat together butter, powdered sugar, and vanilla to make your icing.
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Divide into 3 portions and color each one with a different gel color.
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Drop spoonfuls of each color on top of your cooled brownies in a random pattern, then spread out over the surface to make a wavy, swirled top layer.
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Drop on various sprinkles, then let sit for at least 20 minutes so icing can stiffen up.
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Cut into squares and serve.