Prepare the Blackberry Swirl by cooking 1 cup of ripe blackberries, ¼ cup granulated sugar, and 1 tablespoon lemon juice until thickened. Strain through a sieve to remove seeds.

Preheat your oven to 350°F (177°C) and grease a loaf pan.

In a bowl, whisk together 2 cups of cake flour and ½ teaspoon of salt. Set aside.

Cream 1 cup softened butter and 1 ¾ cups granulated sugar for 5 minutes until pale and fluffy.

Add 1 teaspoon vanilla extract and 3 large eggs one at a time, mixing well after each addition.

Gradually add the flour mixture, alternating with ¾ cup sour cream, mixing just until combined.

Combine ½ cup of the batter with the blackberry puree, then layer in the loaf pan starting with half the plain batter, then the blackberry mixture, and top with remaining batter. Swirl gently.

Bake for 60-70 minutes until a toothpick comes out with moist crumbs.

Cool the cake in the pan for 20 minutes, then transfer to a wire rack.

Prepare the icing by mixing together blackberry juice, 1 cup powdered sugar, 1 tablespoon milk, and a pinch of salt.

Drizzle the icing over the cooled cake and allow it to set before slicing.