Heat oven to 350℉/177℃.
Line 2 baking sheets with parchment paper. Set aside.
With an electric mixer, beat butter and sugar until light and fluffy, about 2-3 minutes.
Add egg, milk, and vanilla.
Beat well.
Add flour, baking powder, and baking soda.
Mix only until combined.
Fold in blueberries.
Use a 1½ Tablespoon cookie scoop to portion dough onto prepared sheet pans.
Press to flatten slightly.
Top each cookie with streusel.
Bake until bottoms are starting to brown, about 15-17 minutes.