Step 1: Preheat the oven to 350°F (175°C).
Step 2: In a large mixing bowl, combine the Greek yogurt and granulated sugar. Whisk until smooth and creamy. Add the zest of 1 1/2 pink grapefruits and mix well.
Step 3: Add the eggs one at a time, whisking after each addition until fully incorporated.
Step 4: In a separate bowl, whisk together the all-purpose flour, baking powder, salt, and vanilla extract. Gradually add to the wet mixture, stirring until just combined.
Step 5: Slowly drizzle in the canola oil, gently folding it into the batter.
Step 6: Pour the batter into a greased 9-inch round cake pan and bake for 25-30 minutes or until a toothpick comes out clean.
Step 7: For the grapefruit syrup, combine the grapefruit juice and granulated sugar in a small saucepan. Heat until the sugar dissolves, then simmer for about 5 minutes.
Step 8: Once baked, cool the cake in the pan for 10 minutes, then transfer to a cooling rack. Brush the syrup over the warm cake.
Step 9: For the glaze, whisk together the powdered sugar and grapefruit juice until smooth, adjusting consistency as needed.
Step 10: Once the cake is completely cooled, drizzle the glaze over the top.