Steps
- Preheat the oven to 350°F / 175°C. Beat ¾ cup of softened unsalted butter with 1⅓ cups of superfine sugar until creamy. Beat in 1 egg and 2 egg yolks.
- Mix 1 cup of warm whole milk, no hotter than 100°F / 38°C, with 2 teaspoons of vanilla extract. Sift 2¼ cups of all-purpose flour, 4½ teaspoons of baking powder, and 1 pinch of salt.
- Mix the flour into the butter in three additions, alternating with the milk in two additions, until smooth.
- Spread in an ungreased 9 × 13in / 23 × 33cm baking dish with 3in / 7.5cm sides. Refrigerate while making the syrup.
- Bring 1 cup of maple syrup, 1½ cups of brown sugar, 3 cups of water, and 1 teaspoon of salt just to a boil in a saucepan.
- Set the baking dish on a baking sheet and gently pour the hot syrup over the batter. Bake for 40-60 minutes, until browned and a toothpick inserted in the cake comes out clean.