Sift cornstarch, salt and brown sugar together. Whisk egg yolks, vanilla and bourbon together.

Put dry ingredients in a saucepan over low heat. Whisk in milk. Increase heat to medium and whisk for 2 minutes.

Add butter and stir constantly with a wooden spoon until it bubbles and thickens, approximately 3 minutes.

Whisk hot mixture into egg yolks. Return to medium heat, stirring for 1 minute. Chill 2 hours.