Prep the Crumble Base: Sift the 2 ¾ cups flour, 1 tsp salt, and 1 tsp baking powder into a large bowl. Cut the 1 ½ sticks of cold butter into small cubes and add them to the flour. Use your fingers to rub the butter into the flour until the mixture looks like coarse breadcrumbs. Stir in ½ cup of the sugar.
Mix the Wet Ingredients: In a separate small bowl, whisk together the 2 eggs, ¾ cup milk, and most of the lemon zest (save a little zest for the very end).
Combine into a Batter: Stir the milk and egg mixture into the dry flour/butter crumbs. Mix until just combined. The batter will be very thick—more like a heavy paste than a pourable cake batter.
Layer the Cake: Spread about two-thirds of the batter into your prepared pan. Scatter half of the Berry Fresh blackberries over the top. Dollop the remaining batter over the berries (it’s fine if it doesn’t cover them completely), then top with the remaining Berry Fresh blackberries and a heavy sprinkle of the remaining ½ cup sugar.
Bake and Cool: Bake for 55–60 minutes until the top is golden and firm. A toothpick inserted into the center should come out clean.
Final Touch: While the cake is still hot, sprinkle the last of the lemon zest over the top. Let it cool completely in the pan before slicing; this “rub-in” style cake needs time to set so it doesn’t crumble when you cut it.
