Preheat the oven to 375 F, and grease your donut pans with non-stick baking spray.

Measure the sugar into a bowl, and add the lemon zest. Use your fingers to rub the zest into the sugar, until very fragrant.

Whisk in the egg, sour cream, milk, vanilla extract, lemon extract and melted butter until smooth.

Separately, sift together the flour, baking powder and salt, then whisk until well combined. Add the dry ingredients to the wet ingredients, and whisk the batter just until combined.

Scrape the batter into a piping bag and cut off a half inch from the pointed end. Pipe the batter into the donut pans, filling them no more than ½ - ⅔ full.You can also do this step with a spoon instead of a piping bag, then smooth out the top of the batter after spooning it into the donut pans.

Bake the donuts on the center oven rack for about 12 minutes, until a toothpick comes out clean. Cool the donuts for several minutes in the pan, then gently remove the donuts and let them cool completely on a cooling rack before adding the frosting.TIP: These baked lemon donuts are delicate when warm, and can break if you try to lift them out of the pan. The best way to remove them from the pan is to place a cooling rack on top, and then invert it to let the donuts gently fall out onto the rack.

While the donuts are cooling, make the lemon sugar to coat the donuts. Measure the sugar into a bowl, and add the lemon zest. Use your fingers to rub the zest into the sugar, until very fragrant.

While the donuts are slightly warm, coat them in the lemon sugar.TIP: If you find that the sugar doesn't stick very well, melt 3 tablespoons butter and use a pastry brush to very lightly brush the butter onto the donuts, then coat the donuts in the lemon sugar.