In a small saucepan, stir together cream and salt. Bring to a simmer over very low heat until salt dissolves. Remove from heat.
In a medium saucepan, stir together sugar and 2 tablespoons water, being careful to not splash onto sides of pan. (Mixture should be the consistency of wet sand.) Cook over high heat. Brush down sides of pan using a pastry brush, stirring mixture to help dissolve sugar. (Do not stir once it starts to boil.) Cook until mixture is a light amber color. Remove from heat. Gradually add warm cream mixture. Add butter, a few pieces at a time, whisking to combine. Add vanilla, stirring well. Let cool completely.
Recipe by TeaTime Magazine at https://teatimemagazine.com/paris-tea-cakes/