Prepare the topping: Whisk together the flour, sugar, cinnamon and salt. Cu in the butter in with a pastry blender, fork or your fingertips until the mixture resembles coarse crumbs. Set aside.

For the cake: In a medium bowl, whisk together the flour, baking powder and salt until combined. Set aside. In a large bowl or the bowl of a stand mixer, beat the butter, sugar and lemon zest together until light and fluffy, about 2-3 minutes. Add the egg and vanilla and beat until combined. Beat in 1/3 of the dry ingredients just until combined. Add 1/2 of the milk. Add another 1/3 of the dry ingredients, followed by the remainder of the milk. Add the remaining dry ingredients and mix until combined. Using a spatula, carefully fold the berries in to the batter.

Spoon batter into prepared pan and smooth a bit so that it is relatively flat. If using nuts, scatter them on top, then sprinkle the prepared topping evenly over-top.

Bake in heated oven for about 40 minutes, or until a toothpick inserted into the middle of the cake comes out clean. Remove from oven and allow to cool in the pan on a cooling rack for at least 15-20 minutes. Run a knife around the inside edge, then carefully loosen and remove the outer ring. You can transfer the cake to a serving plate by loosening the parchment from the metal bottom and sliding on to a serving plate.