1. Preheat your oven to 350°F (175°C) to ensure even baking.
2. Toss fresh or frozen blueberries with a tablespoon of flour and lemon juice in a small bowl to prevent sinking into the batter and create a jam-like layer.
3. Whisk together flour, sugar, baking powder, salt, and cinnamon in a medium bowl to prepare the dry ingredients.
4. In a large bowl, cream softened butter and sugar until light and fluffy. Gradually add eggs, one at a time, mixing well after each addition. Stir in vanilla extract and milk (or sour cream) to complete the wet ingredients.
5. Slowly mix the dry ingredients into the wet ingredients, blending until just combined to create the batter without overmixing.
6. Separate about a quarter of the dry ingredient mix into a separate bowl to create the crumb topping. Add additional sugar and softened butter, mixing with fingers to form a crumbly texture. You can mix in oats for added texture, if desired.
7. Grease or line a baking pan with parchment paper and evenly spread half of the batter on the base of the pan.
8. Layer the prepared blueberries over the batter. Optionally, spread a thin layer of blueberry jam to enhance the flavor.
9. Add the remaining batter evenly over the blueberry layer, then scatter the crumb topping generously on top.
10. Place the pan in the preheated oven and bake for 25-30 minutes until the crumb topping turns golden brown and the bars are set.
11. Allow the Bursting Blueberry Crumb Bars to cool completely before cutting into individual portions for serving.
12. Serve warm or at room temperature, optionally pairing with vanilla ice cream, whipped cream, or tea/coffee for the best experience.