Instructables

Pi within a Pie

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Step 10:

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I then used a makeshift piping bag (filled with the leftover peanut butter filling) to add a circle to the middle of the pie (it's a little hard to see, but it's there!) and a peanut butter edge next to the crust.  These would act as dams and help create the top of the π. 

Once the trenches and dams were done, the pie went back into the refrigerator for 30 minutes or so – it softens really quickly!