Combine the flours, sugar, baking powder, salt and lemon zest in a food processor. Pulse until mixed. Add the egg and vanilla. Pulse until combined. Add the butter and blend until the mixture forms into a dough. Form the dough into a disc, cover with plastic wrap and freeze for 30 minutes.

Place an oven rack in the center of the oven. Preheat the oven to 375 degrees F. Line a baking sheet with parchment paper or a silicone mat.

On a generously-floured work surface, roll out the dough into a 10-inch-diameter circle, about 1/4-inch thick. Using a 3-inch round cookie cutter, cut the dough into 12 pieces and transfer to the prepared baking sheet (any leftover pieces of dough can be gently kneaded together, rolled out and used for more dough circles). Sprinkle the dough with the nonpareil sprinkles. Press the sprinkles gently into the dough. Bake until light golden around the edges, 8 to 10 minutes. Cool for 5 minutes. Transfer to a wire rack to cool completely, about 15 minutes.