Mix up the butter and sugar until really combined. Mix the flour, sumac, baking powder and salt, and then add to the creamed butter slowly, mixing together until a dough is formed. Divide the dough into thirds, wrap in wax paper and place in the freezer to speed chill for 45 minutes.
Transfer the dough to the fridge to keep cool while rolling out. Preheat the oven to 375 degrees F.
On a clean work service, sprinkle some powdered sugar. Also sprinkle some powdered sugar on a rolling pin or wine bottle. Roll out the dough to desired thickness. Work quickly to keep the dough COLD. Keep in mind that the thinner you roll them, the shorter the cooking time. Do you want wafers or cookies? Your choice. Bake these guys, in batches if necessary, on a silicone baking mat on a cookie sheet until they are just golden brown on the edges, 8 to 10 minutes. It might be wise to rotate the sheet halfway through baking, depending on your oven.
Remove from the oven, let them cool and give them to the kids to decorate. Celebrate sumac.