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1
Melt the butter in a heavy saucepan over medium-low heat. Add the shallots in fine brunoise and sweat without colour until fully softened and translucent — they must carry no trace of browning.
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2
Add the white wine and white wine vinegar in equal parts. Raise the heat and reduce until only two tablespoons of sticky, intensely concentrated liquid remain — a near-glaze that coats the base of the pan. This is where the dish lives or dies: an insufficient reduction produces a thin, one-dimensional sauce; the correct reduction is the flavour engine upon which everything else is built.
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3
Add the demi-glace. Bring to a gentle simmer and cook, uncovered, for 15 minutes. The sauce should darken and take on a glossy, unified body. Skim any fat or impurities that rise.
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4
Pass the sauce through a fine chinois, pressing the shallots firmly to extract all liquid. The strained sauce should be dark, smooth, and glossy with no solid matter remaining.
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5
Remove the pan from the heat entirely. Add cayenne pepper one pinch at a time, tasting between each addition. The target heat is a slow warmth that builds on the back of the palate — perceptible but not painful. Add freshly ground white pepper alongside for complexity. The sauce is done when the acidity is sharp and forward and the heat arrives slowly, after the first swallow.