In a small sauce pan, whisk brown sugar and butter over low heat until mixture starts to bubble.
Remove from heat and stir in ½ cup heavy cream. Mixture will bubble up some. Reserve 1/4 cup caramel sauce.
Return caramel sauce to stove and stir in remaining heavy cream, milk, sugar and salt.
Heat over low heat until sugar has completely dissolved.
In separate bowl, whisk the egg yolks. While vigorously whisking, pour ¼ cup hot cream mixture into the yolks to temper the eggs.
Add the egg yolks back into the cream. Continue to cook the mixture while constantly stirring. Once large bubbles start to appear, cook one more minute and then remove from heat.
Strain through sieve into large bowl. Completely chill mixture in the refrigerator.