Heat the oven to 350 degrees, and a 9x9 baking dish well.
In a bowl, beat together the melted butter and sugar until combined well.
Add in the eggs, and then slowly add in the flour, salt and baking powder.
Blend in the lemon zest and vanilla, and then fold in the blueberries.
Spread the batter into the baking dish.
In a separate bowl, mix the ingredients for the streusel until they form a crumbly mixture.
Spread the streusel mixture on top of the cake.
Bake for 40 minutes.
Cool to room temperature before serving.