1: Preheat your oven to the appropriate temperature and prepare your baking pan.
2: In a bowl, combine the all-purpose flour, baking powder, lemon zest, and salt.
3: In another bowl, cream together the unsalted butter and granulated sugar until light and fluffy.
4: Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.
5: Mix in the lemon juice and buttermilk until just combined.
6: Gradually add the dry ingredients to the wet mixture, mixing until smooth.
7: Prepare the crumble by combining the sugar, melted unsalted butter, flour, and pinch of salt in a small bowl.
8: Pour the cake batter into the prepared pan and top with the crumble mixture.
9: Bake in the preheated oven for about 60 minutes or until a toothpick comes out clean.
10: Allow the cake to cool before icing with a mixture of powdered sugar, lemon juice, and milk.
