Grease and flour a 9" square baking tin, or line with parchment.
Preheat oven to 375 F.
Beat together the brown sugar and butter until light and fluffy.
Add the egg and vanilla and beat well.
Stir in the milk.
Sift together the flour, baking powder, and salt. Stir this dry mixture into the butter and sugar mixture.
Spread half the batter in the pan. Evenly sprinkle with half the streusel mixture.
Carefully spread the remaining batter over the streusel. Sprinkle remaining streusel on top.
Bake for 25-30 minutres, or until a wooden skewer inserted in the centre comes out clean.
Let cool and cut into 9 squares.
Serve with coffee (or tea).