For the crumble, add all of the ingredients to a bowl and mix it together, either by hand using a fork or with an electric mixer, until a crumble structure and there is no dry flour left. Set it aside while preparing the cake.

Preheat the oven at 175ºC/350ºF conventional oven and prepare a 23 cm/9-inch springform with parchment paper.

Sift together the all-purpose flour, baking powder and salt. Give it a whisk to make sure everything has been combined and set it aside.

In a large bowl using an electric hand mixer or in a bowl of a stand mixer, using the paddle attachment, add the butter, and granulated sugar and mix it on high speed for 3 minutes.

Add the eggs one at a time into the mixture, and on low speed mix until combined.

Continue by adding half the dry ingredients, on low speed, mixing until just combined.

Then add the lemon zest, sour cream, vegetable oil, vanilla extract and mix until just combined. Then add the other half of the dry ingredients and mix until combined.

If using fresh blueberries, clean and dry the blueberries. In a small bowl toss the blueberries in the cornstarch. Add half of them into the cake batter, and gently fold them in, using a rubber spatula.

Add the cake batter into the prepared pan and even it out. Top it with the remaining blueberries and finish with add the crumble topping.

Bake it for 45-50 minutes or until cake tester comes out clean. Once baked let the cake cool down. After 10 minutes, gently drag a thin knife along the edge of the cake to release the cake from the springform. Let it cool down completely on a wire rack.

Once cooled down and ready to serve, stir together all of the ingredients for the glaze until smooth. Drizzle it over the cake and it’s ready to be enjoyed.