Heat the oven to 110C, 90C fan oven, mark low. With a pencil, mark a 22.5cm square on a sheet of baking parchment and turn over, pencil down, onto a large baking sheet.
Mix together the egg whites, sugar and vinegar in a heatproof bowl. Place over a saucepan of gently simmering water and beat with an electric whisk for 10 minutes or until very stiff and shiny.
Spread the meringue mixture into the marked square and bake for 40 minutes then cool. The meringue will feel soft and mallowy and the top just slightly firm.
Whip the cream until it just holds its shape and fold in half the caramel. Keep whisking slowly for a few more seconds until the cream is just firm enough to spread.
Cut the square into 3 lengths, each roughly 7.5cm x 22.5cm. Using a palette knife, lift one length of meringue onto a serving platter and spread with about a third of the cream. Drizzle with half the remaining caramel and place a second length of meringue on top and spread again with cream. Drizzle with the remaining caramel and top with the final meringue.
Scatter the top with the almonds, dust with icing sugar and add edible snowflakes, to serve.
The assembled meringue terrine without the almond decoration will freeze for up to 1 month. Thaw in the fridge for 2 hours before serving.
Edible snowflakes can be made with ready-made fondant and snowflake stamps/cutters. If you want the snowflakes to harden and stand upright use white flower paste, available from cake decorating shops.