Preheat the oven to 350 degrees Fahrenheit. Grease an 8×8-inch square baking dish.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.

In a separate large bowl, using a handheld electric mixer, cream the softened butter and sugar until light and fluffy. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.

Gradually add the dry and wet ingredients, alternating with the sour cream, then add milk. Mix until just combined. Gently fold in the blueberries.

Spread the cake batter into the prepared baking pan.

Make the streusel topping: In a small bowl, combine the flour, brown sugar, cinnamon, and salt. Add the cold butter and use your fingers or a pastry blender to mix until crumbly. Stir in the chopped pecans to the mixture. Sprinkle over the batter.

Bake in the preheated oven for 45-55 minutes or until a toothpick inserted into the center comes out clean. The streusel topping should be golden brown.

Allow the coffee cake to cool completely on a wire rack.

In a small bowl, whisk together the powdered sugar, milk, and vanilla extract until smooth. If the glaze is too thick, adjust by adding a bit more milk if needed. Once the coffee cake is completely cooled, drizzle over the top.

Slice the blueberry coffee cake into squares and serve.