Lemon Curd

1 1/2 cups sugar
4 teaspoons cornstarch
2 eggs
1 1/2 cups water
3/4 cup lemon juice
3 tablespoons unsalted butter, softened
Batter
1 cup unbleached all-purpose flour
1 teaspoon baking powder
1 pinch salt
2 eggs, separated
1/2 cup sugar
1/4 cup unsalted butter, softened
1/4 cup canola oil
Grated zest of 1 lemon
2 tablespoons lemon juice

Directions

Lemon Curd
1. In a saucepan off the heat, combine the sugar and cornstarch. Add the eggs and whisk until smooth. Add the water, lemon juice and butter. Bring to a boil over medium-low heat, whisking constantly and scraping the bottom and corners of the saucepan. Pour the curd into a 2-litre (8-cup) soufflé dish. Set aside.
Batter
1. With the rack in the middle position, preheat the oven to 190°C (375°F).
2. In a bowl, combine the flour, baking powder and salt. Set aside. In another bowl, beat the egg whites with an electric mixer until soft peaks form. Gradually add half the sugar while beating until very stiff peaks form. In a third bowl, beat the remaining sugar with the butter, oil, lemon juice, lemon zest and egg yolks with an electric mixer until smooth. On low speed, mix in the dry ingredients. Stir 1/4 of the egg whites into the batter. Using a spatula, gently fold the remaining egg whites into the batter.
Spoon the batter onto the warm lemon curd. Bake until the cake is golden-brown and a toothpick inserted in the centre comes out clean. Serve at room temperature.