1/3 cup soy sauce
1/4 cup white miso
2 teaspoons dry mustard powder
1 teaspoon kecap manis (sweet soy sauce)
1/4 teaspoon smoked sweet paprika
Zest of 1 lime
Kosher salt and freshly ground black pepper, to taste
12 tablespoons unsalted butter, cubed and chilled
In a 2-quart saucepan, whisk together soy sauce, miso, mustard, kecap manis, paprika, zest, salt and pepper, and bring to a boil over medium-high heat.
Remove from heat and slowly add butter, a few cubes at a time. Whisk constantly until smooth and all butter is incorporated; season with salt and pepper before serving over steaks.