Preheat the oven to 180°C, fan 160°C, gas mark 4. Line 2 baking trays with baking paper.
Put the butter and brown sugar in a small pan and melt over a gentle heat. Stir in the flour and bring to the boil.
Stir in the crème fraîche and remove from the heat. Add the almonds, mixed spice, peanuts, raisins and Brazil nuts, and mix well.
Spoon tablespoonfuls of the mixture onto the trays, and gently flatten. Leave space between each one, as they will expand as they cook.
Bake for 15 minutes, until golden. Leave the florentines on the baking parchment and lift onto a cooling rack.
Melt the chocolate in separate bowls in the microwave (see Cook's tip below).
Once the florentines have cooled, turn them over and spread the bases with dark chocolate, then drizzle over the white chocolate in thin lines. Chill in the fridge until set, then serve.
Cook's tip: To melt chocolate in the microwave, use a medium heat setting and check the chocolate every 30 seconds, as it can burn very easily.