Place the black peppercorns in a small bag and crush lightly using a rolling pin or mortar and pestle. This releases the aromatic oils without making the sauce overly spicy.
In a small saucepan, melt the butter with sunflower oil and sauté the finely chopped shallots along with the crushed peppercorns. Cook until softened and fragrant, about 2-3 minutes.
Pour in the brandy and let it reduce slightly. Then, add the beef stock and Worcestershire sauce, bringing the mixture to a rapid boil for about five minutes to intensify the flavors.
Lower the heat and add the double cream, stirring to combine. Let it simmer gently until the sauce thickens slightly. Taste and adjust seasoning with extra pepper or salt as needed.
Remove from heat and serve immediately over your favorite steak or oven-baked chips. For extra indulgence, drizzle over roasted vegetables or chicken.