In a small saucepan, combine 1/4 cup (60 mL) of the sugar, grapefruit zest and grapefruit juice. Bring to a boil over high heat. Reduce heat to low and simmer, stirring, for I to 2 minutes or until sugar has dissolved. Remove from heat and stir in butter until melted. Let cool to room temperature.
Preheat oven to 375°F (190°C).
In a large bowl, whisk together flour, baking powder, baking soda, salt and cardamom.
In a medium bowl, whisk together the remaining sugar, the grapefruit mixture and eggs until well blended. Whisk in yogurt and milk until blended.
Add the egg mixture to the flour mixture and stir until just blended.
Divide batter equally among prepared muffin cups.
Bake for 16 to 21 minutes or until tops are golden and a toothpick inserted in the center comes out clean. Let cool in pans on a wire rack for 5 minutes, then transfer to the rack to cool while you prepare the glaze.
Glaze:
In a small bowl, combine confectioners' sugar, grapefruit zest, grapefruit juice and salt until blended. Spoon over warm muffin tops. Let cool.
Notes
You'll need about 2 large grapefruits to yield the zest and juice needed for these incredibly delicious citrus muffins.