Preheat the oven to 375ºF and line two 12-cup muffin tins with paper liners.

In a large bowl, whisk together flour, sugar, baking soda, baking powder and salt.  In a separate bowl, whisk together yogurt, eggs, vanilla, rosemary, oil, grapefruit juice and milk.  Make a well in the dry ingredients then pour in the yogurt mixture.  Use a light hand and a rubber spatula to gently combine the two, and don't over-mix.  Divide batter among paper cups and bake for 18-22 minutes, until tops spring back when gently pressed in the middle. Transfer muffins to wire racks to cool completely.

To make glaze, slowly whisk the sugar into the yogurt and add enough milk as needed to make a dippable consistency (I added a teaspoon for a fairly thick glaze). When muffins are cool, flip them upside-down and dip their tops in the glaze, letting excess drip off before returning them to the wire racks.  Allow about an hour for the glaze to set.  Top with a small wedge of grapefruit, if desired.