Preheat oven to 350 degrees F. Spray a non-stick muffin tin with cooking spray, or line with paper or parchment liners.
Place brown sugar and grapefruit zest in a medium bowl. Use your fingertips to work the zest into the sugar until fragrant. Whisk in the white whole wheat flour, oat flour, baking soda, and salt.
In a large bowl, whisk together the egg, ricotta (or yogurt), milk, maple syrup (or honey), 1/4 cup grapefruit juice and vanilla. Whisk in the melted butter.
Add the dry ingredients to the wet and stir with a spatula until barely combined. Scoop batter into the prepared muffin tin, filling each well about 2/3 full.
Bake for 15-16 minutes, until a tester inserted into the center of a muffin comes out with just a few moist crumbs attached.
Cool muffins in the pan for 5 minutes and the transfer to a wire rack to cool completely.
To make the glaze, whisk together the powdered sugar and 1 tablespoon grapefruit juice in a small bowl. Use a spoon to drizzle the glaze over the muffins. Enjoy!