Preheat oven to 350°F/177°C. Grease and line a 9-inch square pan. Set aside.
Mix party cupcake mix according to package instructions. Pour cake batter into prepared pan. Bake in the preheated oven for 20-25 minutes or until set.
Set aside cake to cool.
In a nonstick pan, mix together yolks, evaporated milk, condensed milk, all-purpose cream and vanilla. Heat over low fire until mixture thickens.
Once the mixture thickens, add in butter. Pour into a container, cover and refrigerate.
Cut square cake in half. On one piece, spread filling, top with other half of the cake.
Refrigerate to set.
Cut into desired portions. Serve.