Make the sablé base

• Crush the sablé biscuits into fine crumbs.
• Mix the crumbs with the melted butter until evenly combined.
• Press the mixture firmly into the bottom of the springform pan and chill.

Prepare the chocolate mousse

• Melt the dark chocolate and let it cool slightly.
• Whip the cold heavy cream to soft peaks.
• Whisk the egg whites with the sugar to a glossy meringue.
• Fold the melted chocolate, whipped cream and meringue together gently.

Assemble and set the mousse cake

• Pour the mousse over the chilled sablé base and smooth the top.
• Freeze until completely firm.

Make the mirror glaze

• Bloom the gelatin sheets in cold water.
• Heat the water, sugar and glucose syrup in a saucepan until dissolved.
• Off the heat, add the bloomed gelatin, condensed milk and white chocolate, then blend until smooth.
• Add the food coloring and blend again, then strain through a sieve and cool to about 32 °C.

Glaze the ornaments

• Place the frozen cake on a cooling rack over a tray.
• Pour the mirror glaze evenly over the top so it runs down the sides.
• Tap the rack gently to remove air bubbles and let the glaze set.

Finish and serve

• Refrigerate until the glaze is fully set.
• Decorate as ornaments and slice to serve.