01 -
Heat the oven to 180°C / 350°F. Line a 20 cm square cake tin. Set aside 60 g flour, 40 g cold butter and 40 g sugar for the topping; the remaining amounts go into the cake.
02 -
Rub the reserved cold butter into the reserved flour and sugar with your fingertips until you have uneven crumbs. Chill this topping while preparing the batter.
03 -
Beat the remaining 110 g softened butter with the remaining 120 g sugar until pale. Beat in the eggs one at a time, followed by vanilla and lemon zest.
04 -
Mix the remaining 180 g flour with baking powder and salt. Fold into the butter mixture in two additions, alternating with the milk. Spread evenly in the tin without overmixing.
05 -
Scatter blackberries over the batter, then distribute the crumble on top. Bake for 45–50 minutes, until golden and a skewer inserted into the cake portion comes out without wet batter; avoid testing through a berry.
06 -
Cool in the tin for 20 minutes, then lift onto a rack and cool another 40 minutes before cutting into nine squares. The fruit stays hot longer than the cake, so allow the full cooling time.
07 -
Cover and refrigerate leftover cake once cool. Eat within three days. Let individual portions lose their refrigerator chill before serving.