For the cake

1. Preheat oven to 325°F/163°C.  Butter and flour a mini loaf pan.  Set aside.
2. In a medium bowl, sift together flour, baking powder, baking soda and salt.  Set aside.
3. In a large bowl using a hand-held mixer or stand mixer fitted with the paddle attachment, beat together softened butter and honey on medium-high speed until light and fluffy, about 4-5 minutes.  Scrape down the sides of the bowl as needed.  Beat in eggs, one at a time, until combined.  Scrape down the sides of the bowl and beat in vanilla and grapefruit zest until mixed well.
4. Combine the grapefruit juice with milk.  With the mixer on low speed and in three parts, alternately add the dry ingredients and the milk mixture to the batter, beginning and ending with the dry ingredients.  Beat until just combined.
5. Fill baking tins 2/3 full and bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.  Remove from the oven and cool on a wire rack for 10 minutes, then remove each loaf and allow to cool completely on wire rack.

For the glaze

1. Whisk the sugar, grapefruit juice, zest and honey in a small mixing bowl until smooth.  Spoon the glaze over the cooled cakes.