Cream the butter and sugar together. Add the eggs one at a time. Add flour and baking powder, then milk, vanilla and salt. Fold in the blueberries. Pour batter into a greased & floured 9-inch (22cm) cake pan.
Mix the sugar, flour and cinnamon in a bowl. Rub in the butter until it reaches coarse meal. Sprinkle over batter and put into pre-heated 375°F (190°C) oven for 50-60 minutes or until the centre of the cake is cooked.