Preheat oven to 325F. Lightly grease a 9" round cake pan; set aside.
In a large bowl, beat the eggs for 2 to 3 minutes on high speed until they're pale yellow.
Add the sugar; beat again.
Add flour and baking powder; mix until combined.
Add milk and vanilla; stir until blended.
Pour batter into prepared pan. Bake 25 to 35 minutes at 325F until a pick inserted in center of cake comes out clean.
Cool (in the pan) on a wire rack for 10 minutes. Remove cake from pan and cool completely. (If necessary, run a sharp knife around the edges of the cake first before removing from pan.) While it cools, prepare the sauce.
In a small saucepan, mix together sugar, cornstarch and salt. Gradually whisk in water.
Cook over medium heat until mixture is clear and bubbly. Remove from heat.
Stir in lemon juice, lemon zest and butter until blended. Serve warm over sponge cake.