Preheat the oven to 425°F. In a large bowl, whisk together the all-purpose flour, granulated sugar, baking powder, and salt.

Add the melted unsalted butter, buttermilk, egg, and vanilla extract to the dry ingredients. Mix until just combined, being careful not to overmix.

Carefully transfer the batter into a plastic baggie or a large piping bag. Snip off a corner and pipe the batter into a greased donut pan, filling each cavity about half full.

Bake for 7 minutes, or until the donuts spring back when lightly touched. Remove from the oven and let them sit in the pan for 5 minutes before transferring to a wire cooling rack.

While the donuts cool, prepare the glaze. Melt the unsalted butter in a medium saucepan over low heat. Add the firmly packed light brown sugar and stir constantly for 2 minutes.

Pour in the milk and stir to combine. Bring the mixture to a boil and boil for 2 minutes, stirring occasionally.

Remove the saucepan from the heat. Stir in the vanilla extract, salt, and powdered sugar until the glaze is smooth and lump-free.

Dip the tops of the cooled donuts into the warm glaze, letting any excess drip off. Immediately sprinkle with your topping of choice, such as chopped almonds or granola, before the glaze sets.