In a medium saucepan over medium heat, whisk sugar and cornstarch.
Add the together the milk, eggs, and egg yolk and whisk until completely smooth.
Place the mixture on the stove over medium heat, and continue whisking constantly until the mixture thickens to a pudding-like consistency, about 7-10 minutes.
Once thick, remove the pan from the heat and stir in the butter and vanilla extract until fully incorporated.
Transfer the custard to a bowl, press plastic wrap directly onto the surface to prevent a skin from forming, and let it cool in the fridge or freezer until it is room temperature. (At least 1 hour)
In a separate mixing bowl, beat the cream cheese until smooth. Once the custard has cooled, add it to the cream cheese and using a hand mixer, mix mix on high speed until fully combined.
In another bowl, whip the heavy cream to stiff peaks, then gently fold the whipped cream into the custard mixture until smooth and fluffy. Set aside to begin assembly.