Before you start, preheat your oven to 200C (390F) and lightly butter or spray with oil a 9" x 9" cake pan.

Add the granulated sugar, brown sugar, all purpose flour and cinnamon to a small mixing bowl. Mix these together until well combined.

Now add the cold, cubed butter to the top of this mixture and use your finger tips to rub the dry mixture into the butter until it resembles bread crumbs (it's ok if there are some clumps of butter still). Set this aside while you make the cake batter. If your kitchen is warm, pop this streusel topping in the freezer.

Now mix the wet ingredients together in a bowl. So add the softened butter, egg, sourdough starter, milk and vanilla extract to a bowl or batter jug and whisk until well combined. Set this aside.

Next, combine the dry ingredients, excluding the blueberries, in a bowl and mix until well combined.

Pour the wet ingredients into the dry ingredients and mix together using a spatula until a light cake batter is formed. Set this aside.

Now if you're using frozen blueberries (as you can see I am in the photos) you'll need to toss these in around 20g of flour before you use them. Do this now and then pop them aside while you assemble the cake.

Now take the cake pan you prepared earlier and spread half the cake batter on the base. Then spread half the blueberries across the cake batter. Spread the rest of the cake batter over the blueberries and then add the rest of the blueberries over this so you should have now used up all of the batter and blueberries.

Sprinkle all of the streusel topping over the top of the blueberries and batter and then place the sourdough blueberry buckle into the oven and bake for around 45 minutes at 200C (390F). You'll know the sourdough buckle is done when the streusel topping is golden brown and a skewer comes out clean when inserted into the middle of the cake.

You can serve this cake warm straight out of the pan. To do this, I just a lasagne server to cut and lever the warm buckle out. If you want to slice it evenly, I find it's best to wait until it's cool and then remove from the pan and slice with a serrated knife.