Bake With Zoha

Crumble:

Preheat oven to 350F (conventional / no fan) and prepare a 9x9" square baking pan with parchment paper
Add the flour, brown sugar, cinnamon and salt to a bowl, and whisk until combined and no lumps remain
Pour in the melted butter. Use a spoon or your hands to mix the butter into the dry ingredients gently. It will become crumbly as you do this; do not overmix, only mix until no dry streaks of flour remain
Place in the fridge 
Cake batter:

Make sure the butter, eggs and sour cream are at room temperature
Add the sugar, butter and oil to a bowl and whisk for 2-3 minutes until light and creamy
Add the vanilla and one egg, and whisk for 20-30 seconds. Add the second egg and whisk again until combined
Add the sour cream and whisk until it is combined. Scrape your bowl, if needed
Sift in the flour, salt, baking powder, and baking soda. Whisk on the lowest speed setting until just combined; do not overmix
Separately, toss the blueberries in lemon zest and flour. Make sure the blueberries are well coated in the flour
Add the blueberries to the cake batter and gently fold until combined (if you want to be extra careful about preventing the blueberries from sinking to the bottom, see my tips in the blog post above!)
Add the batter to the prepared pan and spread it out
Bring the crumble out of the fridge and use your hands to spread it on top. It should be in chunks, and doesn’t have to be perfect!
Bake at 350F for ~45-50 minutes. Test with a toothpick inserted in the middle at the 45 minute mark; it should have light crumbs but no wet batter. If the crumble is becoming too dark, you can cover the pan with aluminum foil as the cake continues to bake
Let the cake rest in the pan for 30 minutes. Then carefully take it out, slice and serve!