Frozen Lemon Squares

This is my mom's recipe, so we grew up with it each summer.  I now also try to make it each summer.  I add a design made of chocolate syrup to the top, before freezing.  I usually buy the crumbs pre-made, which might be the reason that sometimes the crumbs don't stick together well.  I haven't tried making this in a pre-made crust yet. 


1/4 cup margarine or butter, melted
1 1/4 cups graham cracker crumbs
1/4 cup sugar
3 egg yolks
1 14 oz. can sweetened condensed milk
1/2 cup lemon juice
whipped topping (Cool Whip, etc.  I use low-fat/fat free cool whip)


Combine margarine, crumbs and sugar
Press into 8 or 9-inch pan
In bowl, beat egg yolks
Stir in sweetened condensed milk and lemon juice
Pour into crust
Top with topping and freeze