Preheat your oven to 350°F (175°C).
In a mixing bowl, whisk together the flour, sugar, baking powder, and salt. Then, mix in the melted butter and milk. Stir until combined. Pour this batter into a greased 8x8-inch baking dish.
In a saucepan, combine the sugar, cornstarch, and cold water. Heat over medium heat until the mixture becomes thick and clear. Remove from heat and stir in the butter and lemon zest. Pour this syrup evenly over the batter in the baking dish.
Pour the boiling water over the entire dish. Place the baking dish in the oven and bake for about 35-40 minutes or until the top is golden brown.
Let it cool slightly and serve warm. You can also top it with a scoop of vanilla ice cream or whipped cream if desired.