Preheat oven to 350 degrees. In a mixing bowl whisk together flour, baking powder, baking soda, salt and nutmeg for 20 seconds, set aside.
In the bowl of an electric stand mixer whip together butter and sugar until slightly pale and fluffy. Mix in eggs one at a time then blend in vanilla. Add half the flour mixture and mix just until combined then mix in 1/2 of the buttermilk and mix just until combined, repeat process with flour and buttermilk once more. Scrape down sides and bottom of bowl and fold batter several times to ensure it's evenly combined. Pour batter into a large piping bag fitted with a medium round tip. Pipe into greased mini doughnut pans, filling pans about 2/3 full. Bake until toothpick inserted into donuts comes out clean, about 7 - 9 minutes (clean out pan/s and repeat with remaining batter). While donuts are baking whisk together sugar and cinnamon for coating. While donuts are still hot, roll in cinnamon sugar mixture (be sure to roll while hot or the coating won't stick, also if needed you can spray donuts lightly with vegetable oil spray and that helps it stick even better). Once cool store in an airtight container at room temperature.