Make the brown butter. In a small saucepan over medium heat, melt the butter, stirring occasionally. Continue to cook the butter until it becomes foamy and you can hear cracking and popping noises. Once that happens, continue to stir occasionally for 3-5 minutes until the butter develops a nutty aroma, brown bits start to form in the bottom of the pan and the butter turns an amber color. Once this happens, remove from the heat and pour into a mixing bowl. Allow to cool to room temperature, 20-30 minutes.
While the butter is cooling, make the crumb topping. Place the butter in a medium, microwave safe bowl. Heat on high for 30 - 60 seconds or until melted. Add the sugars, cinnamon, salt and flour and stir using a rubber spatula until combined. The mixture will not look very crumbly yet. Spread out mixture on prepared baking sheet to dry until crumbly ready to use.
Preheat oven to 350. Spray a 9 in square baking pan with cooking spray and set aside.
In a medium bowl with a hand mixer or in the bowl of a stand mixer fitted with the paddle attachment, beat browned butter and sugar on medium speed until creamy, about 3-4 minutes. Scrape down the sides of the bowl as needed. Add the egg and beat well until combined.
In a medium bowl, whisk together the flour, baking powder and salt. Gradually add one third of the flour mixture to the browned butter mixture alternately with the milk, beginning and ending with the flour mixture. It should look like this: flour, milk, flour, milk, flour.
In the bowl that had the flour in it, add the blueberries and lemon juice and stir to coat. Gently stir blueberries into the batter. Pour batter into prepared baking pan. Sprinkle with reserved crumb topping.
Bake until golden brown, 40-50 minutes or until a toothpick inserted in the center comes out clean. Let cool in pan for 10 minutes then remove from pan and cool completely on wire rack.