How to Make It
Preheat oven to 375°F. Grease a 12-cup muffin pan with cooking spray. Whisk together flour, sugar, poppy seeds, baking powder, baking soda, and salt in a large bowl until combined.
Whisk together yogurt, melted butter, lemon juice, eggs, 1 tablespoon of the grapefruit zest, and 3 tablespoons of the grapefruit juice in a separate large bowl until smooth. Fold yogurt mixture into flour mixture until just combined (mixture will be thick). Using a 3 oz. ice cream scoop, scoop about 1/3 cup batter into each muffin cup. Place on middle rack in oven, and bake in preheated oven until muffins rise and begin to brown, 20 to 25 minutes. Remove from oven, and cool completely on a wire rack.
Whisk together powdered sugar, Champagne, and remaining 1 tablespoon each grapefruit zest and grapefruit juice in a small bowl until smooth. When muffins have cooled, drizzle glaze evenly over muffins.