Preheat the oven to 350 degrees F (180 degrees C). Prepare two 8x4 loaf pans by greasing and flouring, or lining with parchment paper.
In a large bowl, thoroughly whisk together the All-Purpose Flour (3 cup) and Baking Powder (2 teaspoon).
Zest the Grapefruit (1) until you have 1 tsp zest.
In a separate bowl, use a hand mixer or a stand mixer to combine the Dark Brown Demerara Sugar (1/2 cup), Granulated Sugar (1 1/2 cup), Meyer Lemon Olive Oil (1 cup), Salt (1/4 teaspoon) and Vanilla Extract (1 teaspoon). Beat in the Large Egg (5) one at a time.
Continue mixing for another 3 minutes until the mixture is creamy and light.
Turn the mixer to a low speed and mix in ⅓ of the flour, then mix in half the Milk (1 cup). Repeat with remaining flour and milk, then add in the grapefruit zest.
Divide the batter between the loaf pans and place in the oven to bake for about an hour, or until a toothpick inserted into the center of the cake comes out clean.
Cool on a wire rack for 15 to 20 minutes.
While the cakes cool, prepare the glaze: in a small bowl, whisk together the Powdered Confectioners Sugar (1 1/4 cup), Fresh Grapefruit Juice (1 tablespoon), and Vanilla Extract (1 teaspoon). Add the Milk (1 tablespoon) a little bit at a time until the mixture is smooth but not too thin.
Spread the glaze over the cakes. Slice and serve!