Make The Crumble Topping: Mix the brown sugar, flour, cinnamon, and salt in a small bowl. Mix well. Use a fork or a pastry blender to cut the butter into the mixture. It should be a crumbly, lumpy consistency. Place the bowl in the fridge while you prepare the cake batter.
Make The Cake Batter: You'll need one medium mixing bowl, one large mixing bowl, and an electric mixer.
Oil and flour a nine-inch glass or metal pan, bundt pan, or deep-dish pie pan. Preheat the oven at 350 degrees.
In the medium mixing bowl, cream the room temperature butter and granulated sugar together until it looks creamy. Mix in the eggs, lemon zest, and vanilla. Mix well. Keep the milk handy in a measuring cup.
In the large mixing bowl, mix the flour, baking powder, and salt. With your mixer on medium speed, mix in some of the wet mixtures. Pour in half of the milk and mix it well. Add in the remainder of the wet mixture and mix it well. Add in the rest of the milk and mix until the batter is thick and creamy.
Use a rubber spatula to fold in the blueberries. Mix gently, because you don't want to squash the berries.
Using the rubber spatula, pour the batter into the prepared pan. Take the crumble mixture out of the fridge. Distribute it evenly over the cake batter.
Place the cake pan in the center of the oven and bake for 45 to 50 minutes. Test the cake for doneness. Remove the cake pan and cool on a rack or towel on the counter for 30 to 60 minutes. The cake can then be cut and stored in the fridge wrapped in foil.
