I pour the cold water into a small bowl and sprinkle the gelatin over the surface. I leave it alone for about 3 to 5 minutes. The gelatin absorbs the liquid and forms a soft mixture that dissolves easily later.

I add the beef broth to a small saucepan and place it over medium-low heat. I warm it gently rather than allowing it to boil hard. A gentle simmer gives me better control over the final texture.

Add the fat

I stir in the butter and beef tallow. I continue whisking until both fats melt completely into the broth. At this point, the liquid becomes richer and slightly glossy. I also add the beef drippings if I have them available. Drippings from roasted beef give the sauce a particularly deep character.

I add the bloomed gelatin to the warm mixture. I whisk continuously until no visible pieces remain. I keep the heat low because gelatin does not need aggressive boiling. A gentle cooking temperature helps preserve the smooth texture I want.

I let the mixture simmer uncovered for about 10 to 15 minutes. I stir occasionally as the liquid reduces. The sauce should become slightly thicker than broth. I do not worry if it looks a little loose at this stage because it thickens further as it cools.

I add the sea salt and smoked salt if I am using it. I taste a small spoonful and adjust the salt carefully. For an especially smooth sauce, I pour it through a fine-mesh strainer. This removes any tiny gelatin pieces or solids from the beef drippings.

I transfer the sauce to a clean jar and allow it to cool for several minutes. As the temperature drops, the gelatin firms the mixture. If I want a thinner consistency, I reheat a small portion gently and whisk in a little warm beef broth.