Pi Pie is a type of pie eaten on Pi Day and Pi Approximation Day. It is a way of celebrating the holiday, which is on March 14, because the date is 3/14, which could be interpreted as 3.14, the first 3 numbers of pi.
Pi Pies are usually decorated with numbers or symbols of pie with edible material. People like to eat this while talking about Pi.
Pi Cake is sometimes used as a substitute for Pi Pie for people who either dont like pie, or are allergic to pie ingredients.