Preheat oven to 375F (190C) and grease a 8-inch (top diameter) skillet or similar sized baking dish with high sides. Set on a baking sheet and set aside.
In a medium bowl or the bowl of a stand mixer, cream the butter with the sugar and lemon zest until light and fluffy, about 3 minutes. Scrape down the sides of the bowl as necessary. In a small bowl, beat the eggs lightly with a fork. Add to the creamed butter and sugar mixture and beat to combine.
Stir in the flour, salt and milk and mix just until incorporated.
Spoon the mixture into prepared baking dish, smoothing the top.
Prepare the sauce by stirring together the cornstarch and sugar in a medium bowl. Add the juice of one lemon to a 2-cup measuring cup, then fill 1 1/4 cup (300ml) level with hot water. Add to the bowl with the sugar and stir to combine. Gently pour the sauce mixture over the cake batter.
Bake in preheated oven 35-40 minutes, or until lightly golden on top. Allow to stand in the pan for at least 5 minutes, then enjoy warm with a dusting of powdered sugar or a dollop of ice cream or whipped cream.
If you don't have self-raising flour, substitute all purpose flour, but add 1 1/4 tsp baking powder and 1/4 tsp salt (and omit the "pinch" in the ingredients).