Preheat the oven to 180 °C. Combine the ingredients for the crumble and let the dough rest in the fridge while you prep the cake batter.
Whisk butter, vanilla, sugar and salt until creamy. Add eggs, lemon zest, flour, baking powder and milk and combine to a smooth batter. Pour in a baking tin and put the apricot-halves on top. Cover with the crumble and bake the cake for approx. 50 minutes.