Heat the oat milk in a medium saucepan over medium heat until hot but not boiling.

In a separate bowl, whisk together the cornstarch, egg yolks, and sugar until smooth.

Slowly pour ⅓ of the hot oat milk into the egg mixture, whisking constantly to temper it.

Pour the tempered egg mixture back into the saucepan.

Cook over medium-low heat, whisking constantly, until the mixture thickens to a light, silky pudding consistency.

This should take about 5 to 8 minutes.

Remove from the heat once the pastry cream has thickened.

Stir in the cold lactose-free butter and chopped dark chocolate.

Stir until the chocolate has fully melted and the cream is smooth.

For an ultra-smooth, creamy texture, blend with an immersion blender.

Transfer to a container.

Cover the surface with plastic wrap.

Let the pastry cream cool to room temperature before refrigerating.

Let it set as it cools.

Use to fill pastries or serve on its own.