Preheat oven to 350°F. Cover a 9" x 13" square pan with nonstick cooking spray.In a medium bowl, whisk together flour, baking powder, and salt. In a large bowl, mix together the eggs while adding in the sugar. Mix until Smooth. Stir in Greek yogurt, key lime rum, zest, vanilla, oil, and buttermilk. Fold in the flour mixture. Do NOT over mix. Pour batter into prepared pan and smooth evenly. Sprinkle streusel over batter. Bake in preheated oven for 45 to 50 minutes or until a cake tester inserted into the center comes out clean. Let cake cool in pan for 30 minutes.
Mix together flour, sugar, and salt together. Cut in the butter then add the lime zest and coconut, if using.
- White chocolate glaze:
- Combine butter and white chocolate in a microwavable bowl. Heat on high while stirring every 15 seconds until smooth. Beat in powdered sugar and milk. Drizzle over cooled cake.