Preheat the oven to 350°F. Grease a 9x5 inch loaf pan with butter.
Wash the grapefruits with hot water, zest one grapefruit and juice both. Set the zest and juice aside separately.
In a large bowl, cream together the softened butter, sugar, and vanilla extract until fluffy.
Add the eggs one at a time, mixing well after each addition, until smooth.
Stir in the Greek yogurt, grapefruit zest, and half of the grapefruit juice.
In a separate bowl, whisk together the flour, ground almonds, baking powder, and salt. Gently fold the dry ingredients into the wet ingredients.
Pour the batter into the greased loaf pan and smooth the top.
Bake for 50-60 minutes, or until a wooden skewer inserted into the center comes out clean.
Let the cake cool in the pan for 10 minutes before transferring it onto a wire rack to cool completely.
Whisk together the powdered sugar with 3-4 tablespoons of grapefruit juice until a smooth, thick glaze forms. If you'd so like, add a touch of red food coloring for a pink hue.
Spread the glaze over the cooled cake and let it set before slicing and serving.
