- With the rack in the middle position, preheat the oven to 180 °C (350 °F). Generously butter a 20-cm (8-inch) square baking dish.
- In a bowl, combine the flour, baking powder, baking soda, and salt. Set aside.
- In another bowl, combine the brown sugar, oil, cocoa powder, and vanilla with an electric mixer. Add the eggs, one at a time, and beat until the mixture is well blended. At low speed, add the milk alternately with the dry ingredients.
- Pour the batter into the prepared pan and bake for about 40 minutes or until a toothpick inserted in the centre of the cake comes out clean. Cool completely in the pan.
- Place the chocolate in a bowl. Set aside. In a small saucepan, bring the cream and corn syrup to a boil. Remove the pan from the heat and add the tea. Cover and let steep for 5 minutes. Strain through a sieve over the chocolate and let stand for 2 minutes.
- With a whisk, stir until the ganache is smooth. Add the butter and stir until smooth. Let cool and pour over the cooled cake in the pan.