1. Preheat your oven to 350-375°F and grease a 9×9 inch or 9×13 inch baking pan.
2. In a bowl, whisk together flour, baking powder, and a pinch of salt. Set aside.
3. In a separate bowl, cream together butter and sugar until light and fluffy.
4. Beat in eggs one at a time, followed by vanilla extract.
5. Gradually mix in the dry ingredients, alternating with milk (or sour cream/Greek yogurt for extra moisture).
6. Toss blueberries in a little flour to prevent them from sinking.
7. Fold the blueberries gently into the batter.
8. Spread half of the thick batter into the prepared pan.
9. If layering, sprinkle a portion of the streusel topping over the batter.
10. Spread the remaining batter on top and smooth it out.
11. Prepare the crumb topping by mixing flour, sugar, cinnamon, and butter until crumbly.
12. Sprinkle the crumb topping evenly over the cake.
13. Bake for 40-60 minutes or until a toothpick inserted in the center comes out clean.
14. Let the cake cool in the pan before slicing.
15. Optionally, drizzle with a powdered sugar glaze or sprinkle with nuts.
16. Serve warm or at room temperature for breakfast or dessert.
17. Store at room temperature for a few days or refrigerate for up to a week.