Step 1Combine the butter and the brown sugar in a saucepan over low heat. Simmer 5 minutes, stirring. Add the cream and stir until smooth.
Step 2Combine the cornstarch and salt in a bowl. Stir in the milk until the cornstarch dissolves. Add the cornstarch mixture to the saucepan and cook over medium heat, stirring constantly to prevent burning, until the mixture thickens, 5 minutes.
Step 3Whisk 1 cup of the mixture into the egg yolks, then return to the pan. Add the vanilla and optional espresso or maple syrup. Reduce the heat to low and cook, stirring, 1 minute. Strain and pour into 8 custard cups. Serve warm or cold.