Beat the butter with an electric whisk or in a small blender until pale and fluffy. Finely grind the Szechuan peppercorns using a pestle and mortar. Stir the pepper into the butter with the chilli flakes, then whip in the soy sauce until combined. Spoon into a dish or shape using a waxed wrap or parchment and chill until needed. Will keep for one week. Serve on Asian greens, or melted onto steak.