In a medium saucepan, off the heat, whisk together the egg yolks and granulated sugar until pale and smooth.

Gradually whisk in the whole milk, ensuring it blends seamlessly with the egg mixture. Sift in the cornstarch, mixing until fully combined.

Place the saucepan over medium heat and whisk constantly. Cook until the mixture thickens into a custard-like consistency, about 5-7 minutes.

Remove from heat and stir in the ground nutmeg, cinnamon, and vanilla extract.

Add the butter to the mixture and stir until it melts and creates a glossy finish.

Let the jam cool slightly before transferring to clean jars. Refrigerate until fully cooled and thickened.