Preheat the oven to 375º F (190°c). Grease 9x9 inch (23cmx23cm) square baking pan or line with parchment and grease for extra ease in removing from pan when baked.
Make the cake batter: In large bowl, cream butter, lemon zest, lemon juice and sugar until light and fluffy. Add the egg and vanilla, beat until combined.
In separate bowl whisk dry ingredients for the breakfast cake (flour, baking powder and salt).
Add the buttermilk and the dry flour mixture to the butter/egg batter. Carefully stir the batter until combined, but don’t overmix (it will be a thick batter). Fold in the berries.
Spread batter into the prepared pan. Top the cake with the crumb topping.
Allow the cake to cool completely. If desired, when ready to serve sprinkle with powdered sugar and dollop of whipped topping or more berries!