Preheat oven to 375° F.
Combine the first 5 ingredients in a bowl and set aside.
In a standing mixer (or bowl using a hand mixer), cream the butter and sugar.
Add the egg, buttermilk and vanilla and combine.
Add the flour mixture and mix until just combined.
Spoon into greased and floured donut pan.*
Bake for 10 minutes, or until the tops spring back when lightly pressed.
To make the icing, place all of the ingredients in a bowl and whisk to combine.
Cool.
Drizzle with frosting and serve.
* If you don’t have a doughnut pan, place the dough in a large Ziploc bag, seal the bag airtight and then cut off a 1/2 inch of one corner. Pipe the dough into (circles) doughnuts on a Silpat or parchment-lined baking sheet. Bake for 10 minutes and allow to cool.