Line a large cookie sheet with wax paper. Fill it with one layer of graham wafers. We used around 14 ½ sheets, broken into 4 pieces each along the dotted lines.
In a medium bowl, whisk the sweetened condensed milk with the lemon zest and lemon juice. Stir well.
Spread the mixture evenly over top of the graham wafer layer.
Add another layer of wafers on top.
In a large bowl, mix together the icing sugar, melted butter, vanilla and lemon juice. Add the warm water, a small amount at a time. (You may need to add more or less water to achieve a thick but spreadable consistency.)
Add the lemon zest and mix well.
Spread the frosting on top of the wafers. You can use an angled frosting spatula dipped in warm water to help with spreading.
Refrigerate for at least 1 hour.
Slice into bars and serve!
Store in an airtight container in the fridge for up to 1 week. Freezing makes the bars soggy.