On the largest setting of a cheese grater, shred frozen/ultra-cold butter into bowl with flour.
Add brown sugar to the mixture. Stir to combine and set aside for later.
Cake
Preheat oven to 350°F. In a large bowl, add butter and sugar, beating until well combined and fluffy.
Add vanilla extract and egg. Beat until smooth.
In a separate, small bowl, combine flour with baking powder.
Into the sugar/butter mixture, add in flour/baking powder mixture in small parts, alternating with a small splash of milk. Repeat this process until you run out of milk and flour.
Stir in blueberrries. Grease a 6-inch baking pan with butter. Spread the cake mixture evenly thoughout.the pan.
Spread the streusel topping over the top of the cake batter. Bake cake for 50-60 minutes. When the cake is fully baked, a toothpick should come out of the center clean.