Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.
In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy using an electric mixer.
Beat in the egg and vanilla extract until well combined.
In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.
Gradually add the dry ingredients to the butter mixture, alternating with the sour cream, beginning and ending with the dry ingredients. Mix until just combined.
Gently fold in the fresh blueberries with a rubber spatula.
For the streusel topping, combine the melted butter, granulated sugar, brown sugar, flour, and ground cinnamon in a bowl. Mix until the mixture resembles coarse crumbs.
Drop tablespoonfuls of the cookie dough onto the prepared baking sheets, spacing them about 2 inches apart.
Sprinkle a generous amount of streusel topping over each cookie.
Bake in the preheated oven for 12-15 minutes, or until the edges are lightly golden and the cookies are set.
Allow the cookies to cool on the baking sheets for a few minutes before transferring them to a cooling rack to cool completely.