In a double boiler or metal bowl over simmering water, melt butter. When just melted, add lemon juice and mix. Add Vanilla Frosting Mix and mix well with a rubber spatula until smooth. Turn off heat.
Keep mixing until completely smooth and all the little bumps pop or melt. If not pourable after they pop, add another tablespoon of warm water and mix well to make mixture pour easily.
Once totally smooth and while still warm, place cake on a wire rack over a parchment lined sheet pan to catch the drips (you do not want the bottom sitting in the poured glaze). Pour glaze over cake, starting in the middle and working towards the edge. Sprinkle on lemon zest, if using. Let the glaze set up before moving and take care when moving as to not crack the glaze.
Store frosted cake in refrigerator or under cake dome when cool and frosting has set.
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