Make the crumble topping - In a small bowl, stir the flour and sugar together, then add the melted butter and stir until crumbs form. Place in the refrigerator while you prepare the cookie dough.
Preheat the oven to 350 degrees F and line a cookie sheet or two with parchment paper or lightly grease.
In a large bowl using a handheld mixer (or stand mixer with the paddle attachment), cream together the butter and sugars until creamy and smooth.
Beat in the milk and vanilla until fluffy and combined.
Stop the mixer and add the flour. Sprinkle the baking soda, cornstarch and salt on top of the flour. Turn the mixer on low speed and mix until just combined.
Gently fold in the blueberries until incorporated. If some of them break a little, that's fine, just try to be gentle.
Use a cookie scoop or spoon (about 1 1/2 tablespoons) to scoop the dough and place on the prepared baking sheet. Flatten them just slightly (so you can stick the crumble on better).
Sprinkle a generous amount of the crumble topping on each cookie dough, pressing it into the dough.
Bake for 12-15 minutes, until they are just barely golden on the edges. Let them cool completely before serving. Enjoy!