For the Streusel

In a large bowl combine the brown sugar, flour, oats, cinnamon and salt. Using a pastry blender or clean hands, cut in the butter until the mixture has lumps the size of peas.

For the Cake

Preheat the oven to 350° and Spray an 8-inch square cake pan or a 9-inch round pan with nonstick cooking spray and line the bottom with parchment paper.

Sift flour, baking powder, and salt in small bowl and set aside. With an electric mixer, beat the butter and sugar on medium-high speed until light and fluffy, about 3-4 minutes. Scrape down the bowl with a spatula and beat in the vanilla and lemon zest until combined.

With mixer running at medium speed, add eggs one at a time; scraping down the bowl after each egg (at this stage the mixture might look curdled)

Add the flour mixture and stir the batter with a rubber spatula, scraping the bottom and sides of the bowl until the flour is fully blended. The batter will be very heavy and thick. Using a rubber spatula, gently fold in the blueberries.

Transfer the batter to the prepared pan and spread evenly. Pour the streusel over the top of the cake batter.

Bake until golden brown and a toothpick inserted into the center of the cake comes out clean, about 55-60 minutes. Remove from the oven and cool for 30 minutes before slicing (if you can’t wait for it to cool, just dig in but it will be more delicate and crumbly while it is still hot).