Make sure all the ingredients are room temperature--especially the half-and-half. If they are cool, the mixture tends to separate and the cakes will lose volume.
Beat 2 1/2 tablespoons butter, 1/4 cup sugar, and lemon rind at medium speed with an electric mixer until creamy. Add egg yolks; beat 3 minutes or until pale. Gradually add lemon juice and flour, beating until blended. Slowly add half-and-half, beating until blended.
Beat egg whites at high speed with an electric mixer until foamy. Gradually add remaining sugar, beating until soft peaks form; fold into lemon mixture.
Spread remaining 2 tablespoons butter evenly into 6 (6-ounce) ramekins. Place ramekins in a 13- x 9-inch pan. Pour lemon mixture evenly into each ramekin. Add hot water to pan to a depth of 1 inch.
Bake at 350° for 45 minutes or until lightly browned. Remove from oven; cool in water bath. Remove cooled ramekins from water bath. Cover and chill 8 hours. Loosen chilled cakes from ramekins, and invert onto individual serving plates. Garnish, if desired.