Step 1: Preheat your oven to 180°C (350°F) and lightly grease and flour a 9-inch springform pan. Line the bottom with parchment paper for easy removal.
Step 2: Prepare the Crumble Topping: In a medium bowl, combine 1 cup (120g) all-purpose flour, 1/2 cup (100g) granulated sugar, and 1/2 teaspoon ground cinnamon. Add 1/2 cup (113g) cold unsalted butter, cut into small cubes. Using your fingertips or a pastry blender, work the butter into the flour mixture until coarse crumbs form. Place in the refrigerator to chill while you prepare the cake.
Step 3: Cream Butter and Sugar: In a large mixing bowl, cream together 1/2 cup (113g) unsalted butter (room temperature) and 1 cup (200g) granulated sugar until light and fluffy.
Step 4: Add Eggs and Vanilla: Beat in 2 large eggs, one at a time, mixing well after each addition. Stir in 1 teaspoon vanilla extract.
Step 5: Combine Dry Ingredients: In a separate bowl, whisk together 1 1/2 cups (180g) all-purpose flour, 1 teaspoon baking powder, and 1/2 teaspoon salt.
Step 6: Alternate Wet and Dry: Gradually add the dry ingredients to the wet ingredients, alternating with 1/2 cup (120ml) milk, beginning and ending with the dry ingredients. Mix until just combined; do not overmix.
Step 7: Prepare Blackcurrants: Gently toss 2 cups (approx. 250g) fresh or frozen blackcurrants with 1 tablespoon all-purpose flour. This helps prevent them from sinking to the bottom of the cake.
Step 8: Assemble the Cake: Pour half of the cake batter into the prepared springform pan, spreading it evenly. Sprinkle half of the floured blackcurrants over the batter. Carefully dollop the remaining cake batter over the blackcurrants, spreading gently. Top with the remaining blackcurrants.
Step 9: Add Crumble: Evenly sprinkle the chilled crumble topping over the blackcurrant layer.
Step 10: Bake: Bake for 50-60 minutes, or until a wooden skewer inserted into the center of the cake comes out clean. The top should be golden brown and the edges lightly pulled away from the pan.
Step 11: Cool: Let the cake cool in the pan on a wire rack for 15-20 minutes before carefully removing the springform side. Allow it to cool completely before slicing and serving.