This is a recipe that I found some years ago and adapted for the microwave. Butterscotch Pie has always been a favorite of mine and they don't come any easier then this one! Prep time does not include chilling time in the fridge.
In a large microwave safe bowl, combine the brown sugar, cornstarch, salt, and milk. Combine well, then cook on high in the microwave for 8 minutes. Remove the bowl from the microwave.
Pour a small amount of the mixture into a small bowl. Add the egg yolks to the smaller bowl, mixing until well-combined. Pour the egg mixture back into the large bowl, combining well.
Return the bowl back to the microwave and cook on high for and additional 1-2 minutes, until the mixture has thickened. Remove from the microwave and stir in the butter and vanilla. Stir until the butter has melted.
Pour into the prepared pie shell and chill for at least 2 hours.