Preheat your oven to 350°F and grease a 9x13-inch baking pan. I use butter on the bottom and sides so the edges get that extra golden color. This step takes two minutes but makes cleanup so much easier.

Cream the softened butter and granulated sugar together until it looks light and fluffy, about 2 to 3 minutes. I use an electric mixer because it saves my arm, but whisking by hand works too. This is where air gets incorporated into your cake, so don't skip it even though it takes a bit.

Beat in the eggs one at a time, then add the vanilla powder and mix until everything is combined. Scrape down the sides of your bowl once — trust me on this. The batter should look smooth and pale at this point.

In a separate bowl, whisk together flour, baking powder, and salt. Alternate adding the flour mixture and milk to your butter mixture, starting and ending with flour. Mix on low speed or by hand so you don't overdevelop the gluten — that's what keeps your cake tender.

Gently fold the fresh blueberries into the batter with a spatula, being careful not to crush them. I've learned the hard way that aggressive stirring turns everything purple and mushy. Pour the batter into your prepared pan and spread it even.

Make your streusel by combining brown sugar, cold cubed butter, and cinnamon in a bowl. Use your fingertips to work it together until it looks like coarse breadcrumbs — don't overmix or it gets too paste-like. Sprinkle this generous layer over the entire top of your batter.

Bake for 40 to 45 minutes until the streusel is golden brown and a toothpick inserted into the cake comes out with just a few moist crumbs. Don't panic if it looks a little jiggly in the center — it'll firm up as it cools. Let it rest in the pan for 15 minutes before slicing so everything sets properly.