Add sugar, flour and cinnamon in a small mixing bowl and stir together with a whisk to distribute evenly. Toss in the cold cubed butter and cut with a pastry blender or pinch with your fingers until the texture resembles coarse crumbs. Refrigerate until ready to use.

Preheat oven to 350°F.

In a microwaveable bowl, melt the butter in the microwave for 30 - 40 seconds. Set aside to cool.

Brush a loaf pan with some of the previously melted butter, line the bottom with a piece of long parchment paper and allow the excess to drape over the sides of the pan. Brush the parchment paper with some more melted butter and set aside.

In a small mixing bowl, add flour, sugar, salt, baking powder and baking soda. Mix together with a whisk to distribute evenly.

In a medium mixing bowl, add sour cream, eggs, cooled melted butter, vanilla and mix together with a whisk until creamy.

Add the flour mixture to the wet ingredients and fold with a spatula until just incorporated and smooth.

Lightly fold the blueberries into the cake batter, and transfer it to the prepared loaf pan. Tap the pan lightly against the counter to even out the top.

Sprinkle streusel evenly over the top and bake for 50 minutes until a toothpick inserted in the middle of the cake comes out clean.

Let cool on a wire rack completely. To unmold the cake from the loaf pan, take a paring knife and run it along the edges without the parchment paper to loosen it from the loaf pan. Lift it out of the pan using the parchment paper excess.