Preheat the oven to 35o degrees.

Spray a loaf pan with cooking spray and set aside.

In a small bowl combine the flour, baking powder, and salt.

In a large bowl cream together the butter and sugar.

Add in the eggs and mix until well combined.

Add in the sour cream, grapefruit zest, and vanilla. Mix well.

Pour the flour mixture into the butter and sugar mixture and mix until well combined.

Pour the batter into the prepared pan and bake for 50 minutes or until golden brown and a toothpick inserted into the center comes out clean.

While the cake is baking make the simple syrup by combining the sugar and grapefruit juice in a small pan over medium high heat.

Bring the syrup to a boil and heat for 1 minute, stirring constantly. Remove from heaat.

Once the cake comes out of the oven put the loaf pan on top of a wire cooling rack. Poke holes about an inch deep in the cake.

Pour half of the simple syrup over top of the cake and spread it around until it soaks in.

Run a butter knife around the edges of the pan and turn the cake out onto the wire rack.

Turn the cake right side up and then pour the remaining simple syrup on top and spread it around.

Allow the cake to cool completely then place several thin slices of grapefruit on top.

To make the glaze whisk together the grapefruit juice and powdered sugar. Drizzle over top of the cake and slice to serve.