With electric mixer at medium-high speed, beat butter until creamy. Gradually beat in sugar until light and fluffy. Scrape sides of bowl. At medium speed, beat in egg and vanilla. By hand with a spatula, stir in blueberries.
Scrape batter into prepared pan and smooth gently.
Sprinkle with prepared topping. Bake 45 to 50 minutes, until cake is browned, shrinks from sides of pan and a toothpick inserted in center comes out clean. Transfer to wire rack and cool in pan. Cut into wedges or squares to serve. Makes 10 to 12 servings.