Pre-heat oven to 350°, grease and flour a 9 inch (22cm) square pan.
In a large bowl beat together the butter and sugar until creamy then add the vanilla and egg, beat until fluffy (approximately 3 minutes).
Sift together the flour, baking powder and salt, then add to the creamed mixture alternating with the milk. Beat until smooth, about one minute.
In a small bowl mix together gently the blueberry, sugar and zest.
Spread half the cake batter into the prepared cake pan, top with the blueberry filling and then top with remaining batter. Bake for approximately 45-55 minutes (depending whether you use fresh or frozen berries). Check with a toothpick for doneness. Let cool and sprinkle with powdered sugar and or serve with a scoop of ice cream. Enjoy!