Prepare and bake the yellow cake according to package directions in two greased and floured 9-in. round baking pans. Cool for 10 minutes; remove from pans to wire racks to cool completely.

In a saucepan, combine the sugar, cornstarch, and salt; gradually stir in the milk until smooth. Bring to a boil over medium heat; cook and stir for 2 minutes or until thickened. Remove from the heat. Stir a small amount of hot filling into the egg yolks; return all to the pan, stirring constantly. Bring to a gentle boil; cook and stir for 2 minutes. Remove from the heat. Stir in butter and vanilla. Cover and refrigerate until chilled.

Place half of the whipped topping in a large bowl; add the strawberries. Cut each cake horizontally into two layers. Place the bottom layer on a serving plate; spread with half of the strawberry mixture. Top with a second cake layer; spread with custard. Top with a third cake layer; spread with remaining strawberry mixture. Top with the remaining cake layer and whipped topping. Refrigerate overnight. Garnish with strawberries and mint if desired.