Preheat the oven to 350°F and grease or line a 9x13-inch baking pan with parchment paper.

In a medium bowl, whisk together 2 cups all-purpose flour, 1.5 teaspoons baking powder, and 0.5 teaspoon salt.

In a large bowl, beat 0.75 cup softened unsalted butter with 0.75 cup granulated sugar for about 3 minutes until light and fluffy.

Add 2 large eggs one at a time, beating well after each addition, then mix in 1 teaspoon vanilla extract.

Alternate adding the flour mixture with the combined 0.5 cup brewed coffee and 0.5 cup sour cream, beginning and ending with the flour, stirring just until combined.

Toss 1.5 cups fresh blueberries with 1 tablespoon flour, then gently fold them into the batter.

In a small bowl, combine 0.75 cup flour, 0.5 cup packed brown sugar, 0.5 teaspoon ground cinnamon, and 0.25 teaspoon salt; cut in 0.5 cup cold cubed unsalted butter with a fork until crumbly, then stir in 0.5 cup chopped walnuts or pecans if using.

Spread the batter evenly in the prepared pan, sprinkle the streusel topping over it, and bake at 350°F for 35 to 40 minutes or until a toothpick inserted in the center comes out with moist crumbs.

Allow the cake to cool in the pan for 10 minutes before slicing and serving.