Preheat the oven to 350°F. Spread coconut flakes on a baking sheet and bake for 8 to 12 minutes, stirring once, until toasted. Remove from the oven and set aside.

Increase oven to 400°F. Whisk melted butter, buttermilk, and egg together in a large bowl. In a separate bowl, whisk together cake flour, sugar, baking powder, nutmeg, and salt. Add the dry ingredients to the wet and stir to combine, then fold in the diced peaches.

Spoon batter into 6 lightly greased doughnut pan wells, filling each about two-thirds full. Bake 8 to 12 minutes, until the tops spring back when lightly touched.

Cool in the pan for 1 to 2 minutes, then turn out onto a cooling rack set over wax paper and cool completely.

To make the glaze, microwave all glaze ingredients in a shallow bowl in 10-second intervals, stirring between each, until melted and smooth. Dip cooled donuts into the glaze or spoon it over, then return to the rack to set. Top with toasted coconut.