In a large mixing bowl, cream together butter and lemon zest.
Add sugar and cream again until light and fluffy.
Add egg and vanilla and beat until fully incorporated.
In a separate mixing bowl, combine flour, baking powder, and salt.
Add a third of the flour mixture to the butter mixture and whisk to combine.
Add half of the buttermilk to the butter mixture and whisk to combine.
Repeat steps 4 and 5 until the flour mixture and buttermilk are completely incorporated.
Gently fold in blueberries
Spread batter into a greased 9-inch square baking pan.
To make the streusel, combine flour and sugar in a mixing bowl. Add 2 tablespoons of butter and cut together with a pastry blender until the mixture is in pea-sized clumps. Melt the remaining butter in a separate microwave safe bowl. Add the melted butter to the flour mixture, and stir to combine.
Sprinkle the streusel topping over the breakfast cake batter.
Bake in a 350 for 35-45 minutes. Check for done-ness by inserting a toothpick.
Serve warm. You can also freeze, tightly wrapped for later use.