In a medium bowl, whisk together the 2 cups flour, salt, and baking powder. Set aside.
in a large bowl, use a handmixer to cream together the ¼ cup butter and ¾ cup granulated sugar. Then beat in the egg until thoroughly combined.
Alternately add the flour mixture and the ½ cup milk into the large bowl, mixing between each addition and scraping down the bowl as needed until all ingredients are combined and batter is smooth.
Toss the blueberries with 1 tbsp flour until berries are coated, then fold the blueberries into the batter. It’s okay if some berries break open, but you want to keep the berries in tact as much as possible.
Pour the blueberry buckle batter into the prepared baking dish, then place on the prepared baking sheet and set aside.
Take the cubed, cold butter out of the fridge and pour it into the small bowl with the cinnamon crumble mixture. Use a pastry blender or fork to cut the butter into the mixture until crumbly.
Sprinkle the crumble topping over the blueberry buckle batter, making sure the entire baking dish is covered.
Place baking dish on baking pan in oven and bake at 375F for 40-50 minutes, or until a toothpick inserted into the centerof the buckle comes out clean.
Remove blueberry buckle from oven and let cool completely on a wire rack. Serve blueberry buckle slightly warm or at room temperature.