Preheat your oven to 180°C (356°F). Grease and line a standard 9×5-inch (23×13 cm) loaf pan with baking paper.
Prepare the crumble topping by combining the melted butter, sugar, flour, and lemon zest in a small bowl. Mix until crumbly, then set aside.
In a large mixing bowl, combine the granulated sugar and lemon zest. Rub them together with your fingertips for about 1 minute until the sugar becomes fragrant and slightly damp.
Add the softened butter and canola oil, then whisk until well combined.
Whisk in the vanilla extract, eggs, lemon juice, buttermilk (or maas), and vinegar until smooth.
Add the cake flour, baking powder, baking soda, and salt. Fold the dry ingredients into the wet ingredients until just combined. Do not overmix.
Gently fold in the blueberries using a spatula.
Pour the batter into the prepared loaf pan and spread it evenly.
Sprinkle the crumble topping evenly over the batter.
Bake for 55–65 minutes, or until a skewer inserted into the center comes out clean or with a few moist crumbs.
Allow the loaf to cool in the pan for 10–15 minutes before transferring it to a wire rack to cool completely.
Let the loaf rest completely before slicing for the best texture and clean slices.
