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Heat oven to 160C/fan 140C/gas 3 and lightly butter a 20 x 23 cm shallow baking tin. Sift the flours together, then stir in the sugar, almonds and a pinch of salt. Rub in the butter and vanilla seeds to make fine crumbs, then tip into the tin and press down with the back of a spoon. Put in the freezer for 5 mins, then bake for 35-40 mins. Cool.
For the caramel, put the sugar and 100ml water in a heavy-based pan, heat gently until the sugar dissolves, then turn up the heat until it turns dark amber. Stir in the cream in four additions until smooth and shiny – take care as it will bubble up. Stir in the butter and ½ tsp salt. Pour over the shortbread and cool.
For the topping, melt the chocolate and butter over a pan of simmering water, then pour over the caramel and smooth out with the back of a spoon. Chill until firm, at least 30 mins. Cut into quarters, then 16 pieces. Will keep in the fridge for up to 3 days.