In a small saucepan, whisk together yolks, 1/2 cup sugar, cornstarch, lemon zest and salt. Gradually whisk in milk until smooth. Cook over medium-low heat, whisking constantly, until mixture thickens, about 5 minutes. Remove from heat and whisk in lemon juice. Strain mixture through a fine-mesh sieve into a clean bowl. Cover with plastic (pressing directly onto surface of custard to prevent a skin from forming) and refrigerate until cool, about 30 minutes.
Line bottom of an 8-inch baking dish with 6 graham crackers (break into pieces to fill space, if necessary). Spoon 1/2 of pudding over graham crackers, spreading into an even layer. Repeat with remaining 6 graham crackers and rest of pudding. Cover; refrigerate for at least 30 minutes, or up to overnight.
Just before serving, whip cream with remaining 1 tsp. sugar until it holds stiff peaks. Spread whipped cream in a thin layer over top of cake. Cut into squares and serve.