Streusel- Mix flour, sugars, salt and cinnamon in a small bowl. Pour in melted butter and mix with a fork until crumbs form. Chill in the fridge.
Blueberries -In a bowl, toss the blueberries with lemon juice to lightly coat them. Sprinkle in the sugar and flour, then gently mix until the berries are coated. Set aside.
Vanilla Cake- Preheat oven to 170°C Fan/190°C/375°F. Grease and line the sides and bottom of an 8-inch cake tin with greaseproof or parchment paper.
Position the rack on the third shelf or in the centre of the oven.
Sift flour, baking powder, and salt in a medium bowl. Set aside
Using a hand mixer or stand mixer fitted with a paddle attachment, beat butter, oil, sugar, and vanilla bean paste until light and creamy for 10 minutes, scraping the bowl down occasionally.
Add eggs one at a time, beating well after each addition. Scrape the bottom and sides of the bowl.
Mix in sour cream.
Switch to a rubber spatula. Gently fold in the flour mixture until it's mostly incorporated. Then, add the milk and continue mixing until everything is combined- don’t overmix!
Assemble- Pour batter into the pan and scatter blueberries on top. Cover with chunks of crumb.
Bake for 55-60 minutes, rotating the pan halfway, until a toothpick or skewer inserted into the centre comes out clean.
Let the cake cool in the pan for 10 minutes and then remove it and place it on a cooling rack to cool completely.
