- With the rack in the middle position, preheat the oven to 350°F (180°C). Butter an 8-inch (20 cm) square baking pan. Set aside.
- In a bowl, combine the flour, baking powder, baking soda and salt.
- In another bowl, cream the butter, sugar and cocoa powder with an electric mixer. Add the egg and beat until smooth. At low speed, add the dry ingredients alternating with the milk. Pour the batter into the prepared pan, and place the pan on a baking sheet.
- In a saucepan off the heat, combine the sugar, cocoa powder and cornstarch. Add the remaining ingredients. Bring to a boil, stirring constantly, and let simmer for 10 seconds. Pour over the cake batter.
- Bake for about 45 minutes, or until a toothpick inserted in the centre of the cake comes out clean. (This will take about 50 minutes if you’re using frozen blueberries.) Let cool. Serve warm or cold.