Start by heating 1 1/4 cups of milk with pistachio paste until it just starts to bubble. Set aside to infuse.
Whisk egg yolks with sweetened condensed milk.
Mix remaining milk with cornstarch and add to egg mixture.
Slowly pour hot milk into egg mixture while whisking.
Cook mixture over medium heat until thickened, then stir in butter.
Chill in the fridge for at least 2 hours.