Heat sugar over medium-high heat in a heavy bottomed saucepan, stirring often with a heatproof rubber spatula. When the sugar is melted and smooth, remove spatula and continue to cook, without stirring, until smoke begins to rise. Count to ten, and remove from the heat.

Carefully stir in butter until melted and smooth. Add salt and stir in.

Slowly add cream while stirring -- careful, it'll spatter -- then add milk.

Place over medium-low heat and cook, stirring, until it reaches a temperature of 110°F. Remove from heat.

Whisk egg yolks, and, while whisking, slowly pour in about a cup of hot milk and cream mixture. Return egg mixture to pan.

While stirring continuously, cook custard until it thickens, about 180°F. Stir in vanilla extract.

Pour into a bowl, cover with plastic wrap and chill thoroughly, preferably overnight.

Churn custard in an ice cream maker according to the manufacturer's directions.