First, preheat your oven to 375 degrees Fahrenheit. Get a 9"x9" baking pan ready using the nonstick method of your choice. Nonstick spray is quick and easy, but greasing the pan with butter will make your cake extra flavorful.

In a large bowl (or a stand mixer bowl), cream together the sugar and softened butter. Take care not to overmix! You want to achieve a light and fluffy consistency. When you get that, add the egg and vanilla extract.

While the butter and sugar are mixing, combine the dry ingredients.

Slowly add the flour mixture and milk into the wet ingredients. Add a scoop of the flour mixture, then a portion of the milk, going back and forth between the two. Mix this thick batter until it is just combined. Try to avoid overmixing.

Add the blueberries. If you have been using an electric mixer, switch to the paddle attachment or use a rubber spatula to fold the blueberries in by hand. You don't want to smash the juicy blueberries.

Pour the blueberry buckle batter into the prepared pan.

In a small bowl, create your crunchy streusel topping by combining the brown sugar, flour, and cinnamon. Then, using either a pastry cutter or two butter knives, cut the cold butter into the mixture until it has broken down into small pieces and the texture resembles coarse sand. Sprinkle the crumble evenly over the top of the batter.

Bake for 30 to 45 minutes, or until the top of the cake is a beautiful golden brown. Double check by inserting a toothpick into the center of the cake and confirming that it comes out clean.