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1
Prepare hollandaise by the standard method — yolks reduced over a bain-marie, clarified butter emulsified in gradually — but draw the sauce slightly thinner than normal; the whipped cream will supply additional body. Season with salt, lemon juice, and white pepper. Hold the finished hollandaise at 55 °C: it should read liquid but not hot to a clean finger drawn across a spoon.
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2
Pour the cold heavy cream into a chilled bowl. Whip to soft peaks only — the cream should mound loosely and fall back on itself when the whisk is lifted. This is where the dish lives or dies: cream taken to stiff peaks will not integrate; it will produce a mousse-like texture that no amount of folding can correct, while cream left under-whipped will add no lightness at all.
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3
Fold the whipped cream into the hollandaise in two additions, using a large balloon whisk in broad, gentle strokes. After each addition the cream should disappear fully into the sauce. The colour shifts from deep gold to pale yellow; the texture changes from heavy to almost cloud-like. Stop folding the moment the cream is no longer visible — over-folding deflates the cream and produces nothing more than hollandaise with unwhipped cream stirred in.
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4
Assess the finished sauce: tilt a spoon of mousseline and observe the flow. It should move in a slow, thick ribbon — not hold its shape on the spoon, not run freely off the edge. The texture suggests heavy cream barely whipped: liquid but with distinct body. Taste for salt and lemon; adjust in drops. Serve immediately, or hold for no more than ten minutes before the cream begins to deflate.