Heat 2 cups whole milk and ⅔ cup heavy cream in a saucepan over medium heat until hot but not boiling.
In a bowl, whisk 4 large egg yolks, ½ cup strong espresso, ⅓ cup brown sugar, and Pinch of salt.
Sift in ¼ cup cornstarch and whisk until smooth.
Slowly add a splash of warm milk to the egg mixture while whisking.
Pour everything back into the saucepan through a sieve.
Cook over medium heat, stirring constantly, until thickened.
Continue stirring for 1–2 minutes, then remove from heat.
Pour into a bowl and let cool, stirring occasionally.
Chill in the refrigerator before serving.