Preheat your oven to 350℉. Grease an 8x8" or 9x9" baking dish.
In a small bowl, mix the flour and baking powder.
Cream the butter and sugar together. Add in the egg and beat until it's mixed well, then stir in the lemon juice and zest.
Mix about a third of the flour m ix into the butter/sugar mix until combined. Add in half the milk and mix. Alternate between half of the remaining flour, the rest of the milk, and the rest of the flour until it's finally all mixed.
Pour your batter in an even layer along the bottom of your baking dish.
Make the sauce by stirring together the lemon and cornstarch until it's fully mixed. (I used a milk frother to do this very quickly, but you don't need to.) Add in the boiling water and stir it.
Sprinkle the sugar over the top of the cake, then carefully pour the sauce over the top. Try to pour slowly and from low and close to avoid carving a divot into the batter.
Carefully transfer the baking dish to the oven without spilling, and bake for 35-40 minutes. You're looking for the top to start to brown and crack a little.
Let the cake cool for around 15 minutes, and serve while warm with a topping of powdered sugar and some fresh fruit.