Preheat oven to 350°F.
Grease your pan thoroughly with butter. I used a springform cake pan for ease of access, but an 8x8" baking dish works fine.
Toss the blueberries in the two tablespoons of flour to coat them. This helps prevent them from just sinking to the bottom of the cake during baking.
In a bowl, combine the flour, baking powder, and salt.
In your mixer's bowl, cream together the softened butter, sugar, egg, and lemon extract. Mix until smooth.
Slowly add the flour mix and milk to the creamed wet ingredients, a little at a time, mixing until just combined.
Gently fold the blueberries into the batter.
Spread the batter evenly in the cake pan. Don't shake to settle, or you'll sink the berries. Just spread it as evenly as you can.
In another bowl, combine the topping ingredients (cold butter, brown sugar, flour, and cinnamon) until it achieves the texture of wet sand.
Sprinkle the topping over the top of the batter.
Bake for 40-45 minutes. You know the drill; test with a skewer or knife and keep it going if any batter sticks to it.