Preheat the oven to 180°C (350°F). Grease and line the 8-inch loaf pan with baking paper.

Beat the 150 g unsalted butter with the 150 g granulated sugar until the mixture is light and fluffy. Beat in the zest of 1 lemon.

Beat in the 1.5 teaspoons vanilla extract. Add the 3 eggs one at a time, beating well after each addition.

Sift the 150 g all-purpose flour and 1.5 teaspoons baking powder together. Set the dry mixture aside.

Fold half of the sifted dry mixture into the wet mixture. Fold in half of the 60 ml milk until just combined.

Fold in the remaining dry mixture. Fold in the remaining milk to make a thick, smooth batter without overmixing.

Toss the 250 g blueberries with 1 tablespoon all-purpose flour for dusting. Spread half of the batter into the prepared loaf pan.

Arrange the floured blueberries in one even layer across the center of the batter. Cover the blueberries with the remaining batter.

Rub the 50 g cold butter into the 50 g brown sugar and 50 g all-purpose flour for crumble. Sprinkle the crumble evenly over the batter.

Bake the loaf for 40 to 45 minutes at 180°C (350°F). Test the center with a skewer; it is ready when the skewer comes out clean.

Cool the cake in the pan for 15 minutes. Transfer it to a cooling rack and cool completely before slicing.