Melt 4 tablespoons of the butter in a large saucepan. Add the shallots and garlic and cook over moderately low heat, stirring, until softened but not browned, about 8 minutes. Add the vinegar and cook over moderately high heat until evaporated. Add the wine and cook, stirring occasionally, until reduced to about 1/2 cup, about 15 minutes. Add 2 cups of the <a href="/recipes/rich-chicken-stock-strode">Rich Chicken Stock</a> and boil gently until reduced to 3/4 cup, about 20 minutes. Add the cream and boil until the sauce has reduced to 2 cups, about 15 minutes. Strain the sauce through a fine sieve, pressing hard on the solids. Return the sauce to the pan and season with salt and white pepper. Whisk in the remaining 2 tablespoons of butter.