In a small saucepan, heat the half-and-half and heavy cream to a simmer over medium heat, then remove from the heat.
Add the white chocolate and stir until it is fully melted and the mixture is smooth.
Add the butter, salt, and vanilla and stir until fully melted and incorporated.
Pour the sauce into a Mason jar or heatproof airtight container. Use the sauce right away, or let it cool completely, cover and store in the fridge for up to 2 weeks.