Add the dry ingredients, stirring just until flour disappears. Transfer mixture to a gallon size plastic zippered bag. Seal bag pressing out any air. Snip about ¼ inch off of one bottom corner of the bag. Gently squeezing the bag, fill donut wells with batter, evenly dividing between the 12 wells.
Place in oven and bake for 15 to 18 minutes, or until a cake tester inserted into the center of one of the donuts comes out clean and the top springs back slightly when touched.
) Remove the doughnuts from the oven, cool in pan for 5 minutes, then invert onto a wire cooling rack, bottom side up. (The nice rounded bottom will now become the top.)
While the doughnuts are baking, prepare the icing. Place butter in in medium size microwave-safe container. Be sure it’s not too small since the butter will bubble up as it’s browning. Place in microwave and cover container a microwave safe plate (big enough to cover the top). Cook on high for 3-4 minutes or until deep golden brown. (My microwave takes 3½ minutes.) Watch carefully for the last minute as butter can go from browned to burned very quickly.
Remove browned butter from microwave and allow to cool for 5 minutes, then add brown sugar, buttermilk and maple syrup. Stir with a whisk until well combined. Place in microwave and cook on high for 1½-2 minutes or until bubbly and smooth, stopping every 30 seconds to whisk. Take a tiny taste. If it’s still “grainy” return to microwave for another 10-20 seconds.
Add vanilla and 1¼ cups powdered sugar. Continue to whisk until smooth and silky. Mixture should be thick enough to adhere to donuts with a fairly thin coat. Add a bit more powdered sugar if needed.
Dip each donut into frosting mixture. covering ¾ of surface with icing. Turn over, allowing excess icing to drip back into bowl, then set back on the cooling rack. Repeat with remaining donuts. If icing starts to get thick, return to microwave for a few seconds. You can also add a drop or two of additional buttermilk.
Drizzle a bit of extra icing over donuts with a fork, if desired. Allow icing to set for at least 15 minutes before serving (if you can stand it).