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1
Fit a confectionery frame (cadre) on a silicone mat or lightly oiled marble slab. The frame must be set to yield a depth of exactly 15 mm — measure with a ruler before the purée goes in.
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2
Combine the fruit purée, sugar, and apple pectin in a heavy-based copper or stainless saucepan. Stir to distribute the pectin before heat is applied; undissolved pectin will produce an uneven set.
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3
Bring the mixture to a vigorous boil over medium-high heat, stirring constantly with a silicone spatula to prevent scorching on the base. This is where the dish lives or dies: clip a probe thermometer accurate to 0.5°C to the pan and cook, without deviation, to exactly 107°C. Below this temperature the paste will not set firm; above 110°C it becomes hard and brittle. Remove from heat the instant 107°C is reached.
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4
Pour the cooked paste immediately into the prepared frame. Spread to fill the frame evenly — the surface should be level and glossy, with no air pockets.
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5
Leave to set, uncovered, at room temperature for 24–48 hours. The paste is ready when it holds a clean edge when pressed at its centre and no longer feels tacky to the touch.
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6
Lift the set slab from the frame. Using a sharp, thin-bladed knife wiped clean between cuts, cut into 2.5 cm squares. The cut face should be smooth and deeply coloured throughout, showing no pale or sticky interior.
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7
Roll each square immediately in crystallised sugar until all six faces are evenly coated. The sugar should adhere in a distinct, crunchy layer — not dissolve into the surface. Arrange in a single layer on wax paper to dry for one hour before storing.