Preheat oven to 375 degrees. Butter a 9-inch square pan. In a large mixer bowl, beat butter and sugar until well combined. Add milk and egg and beat again.
In a small bowl, combine flour, baking powder and salt. Add to butter mixture and stir with a spatula until combined. Fold in blueberries and spread batter into prepared pan.
For topping, place all ingredients in food processor and process until combined. Sprinkle evenly over batter.
Bake cake for 40 to 50 minutes or until cake tester inserted in center comes out clean. Cool 20 to 30 minutes before serving or cool completely and freeze, wrapped in foil. Bring to room temperature before cutting into squares. |