1. Preheat oven to 350°. Coat 24 mini donut cups with cooking spray.
2. To prepare donuts, place granulated sugar and butter in a large bowl; beat with a mixer at medium speed until well blended (about 5 minutes). Add egg, beating well. Weigh or lightly spoon flour into dry measuring cups; level with a knife. Combine flour and next 4 ingredients (through nutmeg), stirring well with a whisk. Add flour mixture and buttermilk alternately to sugar mixture, beginning and ending with flour mixture.
3. Spoon dough into a heavy-duty zip-top plastic bag. Snip a small hole in 1 corner of bag. Squeeze dough into prepared donut cups, filling two-thirds full. (Do not cover centers of donut cups.) Bake at 350° for 8 minutes or until a wooden pick inserted in donuts comes out clean. Cool donuts in pans on a wire rack 1 minute. Remove from pans. Cool completely on a wire rack.
4. While donuts bake, prepare glaze. Combine powdered sugar, milk, and maple syrup in a small bowl, stirring with a whisk until smooth. Dip top of 1 donut in glaze; place donut, glazed side up, on a cooling rack. Immediately sprinkle colored sprinkles over glaze, if desired. Repeat procedure with remaining donuts and glaze. Let stand until glaze is set.