Combine sugar, cornstarch, salt, cranberry juice, lemon juice, egg, and yolks in a medium heavy saucepan. Cook over medium heat, whisking constantly, until the mixture becomes very thick, 5 to 7 minutes. Continue to cook and stir until the mixture boils, then stir for 30 seconds more. Transfer to a bowl over an ice bath to cool; add food coloring. Stir occasionally until completely cool, then cover with plastic wrap, pressing it onto the surface to prevent a skin from forming, and store in the fridge (filling can be made up to a day in advance and stored in the fridge).

Trim domed tops from cakes and place one on a serving platter; spread cold lemon filling over the surface 1/2 inch from the edge. Top with second cake and place in the fridge while preparing frosting.

Combine 1 cup sugar, lemon juice and corn syrup in a small heavy saucepan over medium-high heat. Stir until sugar dissolves; remove spoon. Brush insides of pan with a wet pastry brush to remove any sugar crystals and place a candy thermometer into the mixture. Boil (do not stir) until the syrup registers 230 degrees Fahrenheit, about 2 minutes. While the syrup cooks, whisk the egg whites and remaining 2 tablespoons sugar with an electric mixer until soft peaks form. With the mixer on low speed, pour the sugar mixture into the egg whites in a slow, steady stream. Increase speed to high and beat until the mixture is thick and glossy, 5 minutes. Tint frosting with food coloring, then frost cake immediately. Garnish with Candied Lemon Zest.