Preheat the oven to 350 degrees F. Use 1 tablespoon of the butter to grease a 10-inch cast-iron skillet.
Whisk together the flour, baking powder, salt and cinnamon in a medium bowl. In another bowl, beat together the sugar and remaining butter with an electric mixer on medium-high speed until light and fluffy, about 2 minutes. Add the vanilla and egg and beat to combine. Add the flour mixture in 2 batches, beating slowly after each addition to incorporate.
Scoop the dough into the prepared skillet and press it in evenly with your fingers. Decorate with the sprinkles as desired. Bake until the edges of the cookie are golden brown and a toothpick inserted into the center comes out clean, about 40 minutes. Cool on a rack. Serve warm or at room temperature.