Line a 8x8 square baking dish with plastic wrap. Set aside.
In a heavy bottom saucepan, heat the butter over medium heat.
When melted, stir in the brown sugar and milk, stirring to combine. Bring to a simmer. When you start to see bubbles, time about 3 minutes, stirring constantly. Mixture should be smooth. Do not overheat, it will seperate.
Remove from the heat and whisk in the peanut butter and vanilla.
Pour the peanut butter mixture over powdered sugar in a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment. You will get the best results using an electric mixer versus hand mixing and pre-sifting your powdered sugar.
Mix until smooth, approximately 4 minutes.
Pour into the prepared square baking dish. Pat mini chocolate chips into the top of half of the fudge.
Cover with plastic wrap and refrigerate until firm, approximately 8 hours.
Cut and serve. I prefer to keep my fudge chilled until ready to serve.
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