Crush 200 g digestive biscuits to fine crumbs in a zip bag with a rolling pin. Stir in 90 g melted butter and a pinch of salt until it looks like wet sand. Press the mixture firmly into the bottom of a 20 cm / 8-inch springform pan with the flat bottom of a glass, then chill for 15 minutes while you make the puree.

Combine 450 g thawed berries, 110 g sugar, and 30 ml water in an 18 cm / 7-inch saucepan. Simmer over medium-low heat for 8 minutes, stirring now and then, until the berries burst and the liquid turns glossy orange.

Press the berries through a fine-mesh sieve into a bowl, scraping the underside, until only seeds and skins are left. You should have about 290 g puree. Weigh out 220 g for the mousse and set 70 g aside for the glaze.

Let the 220 g puree cool for about 15 minutes, until it feels lukewarm at 35 C / 95 F. Meanwhile, sprinkle 8 g gelatin over 40 ml cold water and leave it for 5 minutes to bloom. Microwave for 10 seconds until clear, then whisk it into the lukewarm puree. Let it stand 5 more minutes at room temperature.

Whip 250 ml cold cream, 120 g mascarpone, and 40 g powdered sugar in a large bowl with a hand mixer for about 2 minutes, to soft peaks that droop slightly from the beaters. Fold a third of it into the puree to loosen it, then fold in the rest in two additions until no orange streaks remain.

Scrape the mousse over the chilled base and smooth the top with the back of a spoon. Tap the pan gently on the counter to settle it. Chill for 4 hours, until the edges are firm and the center barely wobbles when you nudge the pan.

In the last 15 minutes of chilling, sprinkle 2 g gelatin over 2 tsp cold water and leave for 5 minutes. Warm the reserved 70 g puree in a small bowl in the microwave for 15 seconds, stir in the gelatin until dissolved, and cool for 5 minutes until barely warm.

Pour the glaze gently over the set mousse, tilting the pan so it covers the surface in a thin layer. Chill for 1 hour, until the glaze is firm. Run a thin knife around the edge, release the springform, and let the cake stand 10 minutes before slicing into 8 pieces.