For the muffins:
¼ cup granulated sugar
2 tablespoons grapefruit zest
1 cup oat flour
¾ cup all-purpose flour
½ cup whole wheat flour
¼ cup brown sugar
1 teaspoon baking powder
1 teaspoon baking soda
¾ teaspoon kosher salt
1 tablespoon poppy seeds
6 tablespoons unsalted butter, melted
¾ cup milk
½ cup greek yogurt (or sour cream)
1 large egg
1 teaspoon vanilla extract
For the glaze:
1 cup confectioners’ sugar
2-4 tablespoons grapefruit juice
To finish:
2 tablespoons granulated sugar
1 tablespoon grapefruit zest
Instead of buying oat flour, you can make your own using rolled oats. Simply use a food processor or, my choice, a coffee grinder, to pulse the oats into flour. Quick tip: to clean your coffee grinder out between different things, just pulse a little uncooked rice in there and wipe it out. Works like a charm.
Preheat the oven to 350º. Grease muffin tins or line with paper liners. In a large bowl, combine the granulated sugar and grapefruit zest. Rub together with your fingertips until the sugar is moistened and fragrant. Add in the flours, brown sugar, baking powder, baking soda, salt, and poppy seeds. Whisk to blend.
In a liquid measuring cup, combine the melted butter, milk, yogurt, egg, and vanilla. Whisk until well blended. Pour the liquid mixture into the bowl with the dry ingredients and fold together just until incorporated and no dry streaks remain.
Use a cookie scoop to transfer the batter into the prepared muffin pans, filling each about ⅔ of the way full. Bake, rotating halfway through, until the muffins are lightly browned and just set, about 20-22 minutes total. Let cool in the pans about 5 minutes, then gently remove from the pans and transfer to a wire rack to cool completely.
Place the wire rack with the muffins over a rimmed baking sheet. In a small bowl, combine the confectioners’ sugar with 2 tablespoons of the grapefruit juice. Add more juice as needed a teaspoon at a time until the glaze is thick but can still be drizzled. You don’t want it too thin or it will soak into the muffins. Drizzle some over each of the muffins.
In a small bowl, combine the granulated sugar and grapefruit zest and rub together until well combined. Sprinkle a bit on top of each muffin.
If freezing the completed muffins, place on a parchment lined baking sheet and freeze until firm. Transfer to an airtight container to store. Warm in the microwave on the defrost setting when ready to serve.