Preheat oven to 350 degrees F. Spray muffin tins with cooking spray (with flour works great!) and set aside.
Whisk together flour, baking powder, salt, and poppy seeds in a small bowl and set aside.
In the bowl of a stand mixer, combine the granulated sugar and grapefruit zest using your fingers to mash together until zest is evenly distributed in the sugar. Add in the melted butter and mix until combined, the add the egg and vanilla; mix to combine and then stir in milk.
Add in the dry ingredients and gently fold the mixture together using a rubber spatula, stirring only until mixture is evenly combined.
Spoon the batter into the molds of your prepared pan, filling each mold up 2/3 of the way. Batter will be very thick. Bake for 10-11 minutes, or until firm and lightly golden on the tops.
Remove pan from oven and allow to cool for at least 10 minutes in the tray before transferring to a cooling wrack to cool completely. Prepare the glaze while they are cooling.
Once cool, dip each donut in glaze and allow to sit for a few minutes for the glaze to set.