Preheat oven to 350° F. 

Spray 4-8 mini bundt pans or one regular bundt pan with non-stick spray or butter. Dust with flour and shake out excess (try to get into all crevices of the pans).

In a stand or electric mixer, beat butter and sugar until soft and light. Grate in the zest of the grapefruit and beat until blended.

Add in eggs one at a time. Slowing down the mixer add in baking soda, baking powder, poppyseeds, and flour.

Add in juice from 1/2 grapefruit and milk, and mix until blended.

Pour batter 1/2 full into cake pans and place on a baking sheet.

Bake for 18-25 minutes (for mini bundts and 45-50 minutes for regular bundts) until cakes are cooked through (test with a cake tester, toothpick, or knife and if it comes out clean it's done).

Let cool for 15 minutes and turn out cakes onto a wire rack to cool completely. (If the cakes are well greased and fully cooked they should turn out no problem).

Combine icing sugar and 1 tbsp of grapefruit juice. If it's still too thick to drizzle add another tbsp of juice.

Drizzle icing over cooled cakes and serve.