Heat oven to 350 degrees F. Grease 2 regular-size muffin tins with baking spray or line 14 muffin cups with paper baking cups.

In a large bowl or bowl of a stand mixer, rub sugar and grapefruit zest together with fingers until well-combined and fragrant. Add oil, poppy seeds, grapefruit juice, vanilla and egg. Beat on low speed until just combined.

In a separate large bowl, whisk together flours, baking powder and salt.

To grapefruit-poppy seed mixture, alternately stir in flour mixture and milk, beginning and ending with flour mixture, until just combined. Pour batter into prepared muffin tins. Bake 20 minutes until a toothpick inserted in center of a muffin comes out clean. Transfer muffins to a cooling rack to cool completely.

In a small bowl, combine ingredients for glaze, adding more grapefruit juice or powdered sugar as needed to achieve desired consistency. Dip top of each muffin in glaze, then return to cooling rack (or a sheet of parchment paper) so glaze can drip and set.