To make the streusel topping, put all the ingredients in a bowl, mix everything using two butter knives until roughly combined, then chill in the fridge until needed.
Heat the oven to 195C (175C fan)/380F/gas 5½ and grease a medium-sized (20cm x 30cm) baking dish with saved butter wrappers or oil.
To make the cake batter, beat the softened butter with the sugar until light and fluffy. Add the flour, sea salt, baking powder, vanilla extract and eggs, then mix until well combined.
Fold in the grapes (or other fruit) and pour the batter into the baking dish. Sprinkle the streusel topping all over the top and bake for 45 minutes to an hour, or until a knife skewered into the cake comes out clean.
