Preheat oven to 325°F (160°C). Prepare an 8-inch cake pan by lining it with parchment paper.
In a large bowl, beat egg yolks with water until smooth. Gradually add sugar, beating until fluffy. Mix in lemon zest, lemon juice, and melted butter.
Stir in flour until evenly combined. Gradually add milk, stirring continuously.
In a separate bowl, beat egg whites with cream of tartar until stiff peaks form. Gently fold into the lemon mixture.
Pour batter into the prepared cake pan. Bake for 40-45 minutes, or until the top is golden and the center is set.
Let cool in the pan for 10 minutes, then transfer to a wire rack to cool completely. Dust with powdered sugar before serving.