Whisk together in a medium saucepan:

Whisk in until smooth:

Whisk in:

Cook over medium-low to medium heat, stirring constantly with a silicone spatula and scraping the bottom and corners of the pan to prevent scorching. When the mixture comes to a simmer and thickens, continue to cook, stirring briskly, for about 30 seconds. Transfer the filling to a medium bowl, straining it through a medium-mesh sieve, if desired. Place a piece of plastic wrap directly on the surface of the filling and refrigerate until chilled. Stir gently before using, but do not beat. This keeps, refrigerated, for up to 2 days.