Preheat oven to 350°F. Coat a 9" springform pan or a 9" x 13" square pan with nonstick cooking spray.
To make the streusel:Stir together flour, sugar and salt together. Cut in the cold butter using two forks or clean hands, then add the lime zest and coconut, if using.
For the cake: In a medium bowl, whisk together flour, baking powder and salt; set aside. In a large bowl, beat together the eggs, then add sugar and mix until smooth. Add Greek yogurt, key lime juice, zest, vanilla, oil and buttermilk, beating well after each ingredient. Gently fold in the flour mixture, stirring until just incorporated. Be careful NOT to overmix.
Scrape batter into the prepared pan and smooth the top. Sprinkle streusel evenly over the batter.
Bake in preheated oven for 45 to 50 minutes, or until a cake tester inserted into the center comes out clean. Allow the cake to cool in the pan for 30 minutes before removing the sides of the springform pan.After 30 minutes, transfer to wire rack to cool completely.
If you've baked the cake in a 9" x 13" pan, serve it right from the pan.
For the white chocolate glaze: Combine butter and white chocolate in a microwave safe bowl. heat on high, stirring every 15 seconds until smooth. Gradually beat in powdered sugar and milk until desired consistency. Drizzle over cooled cake.