Combine the stock, grapefruit juice, and sugar in a shallow heavy-bottomed saucepan. Cook to the caramel stage; there will be wisps of smoke coming from the center of the pan. Carefully whisk in the cream. Allow the mixture to boil and deepen in color. Reduce the mixture to 1 or 1 1/2 cups, whisking constantly. When it is caramel-dark, add the soy sauce. Strain through a fine-mesh strainer and reserve.
Yield: 1 to 1-1/2 cups