Preheat the oven to 180°C, fan 160°C, gas 4 and line one or two baking trays with baking paper.
Put the butter, sugar and 50ml cold water in a saucepan. Bring to the boil, then cook over a high heat for around 5 minutes or until the mixture just begins to turn a pale caramel colour around the edges. Don't stir it; just remove from the heat, and leave to cool for a few minutes. Add the crème fraîche, almonds and vanilla seeds to the pan and gently stir through.
While the mixture cools to warm, separate the egg and whisk the white in a clean bowl until it begins to froth, then fold through the almond mixture.
Spoon the mixture onto the tray/s in small mounds and spread with the back of a spoon to make 12 circles about 2-3cm apart.
Bake for about 15-20 minutes or until a rich golden colour all over. Carefully slide the baking paper/s onto a cooling rack. Leave until the florentines are cold and firm.
Melt the dark and white chocolates in separate glass bowls set over simmering water. Cover the undersides of the florentines evenly with the dark chocolate. Leave for a few minutes before dragging the prongs of a fork down the undersides. Pour the white chocolate into a piping bag and pipe lines across the dark chocolate. Leave chocolate-side up on a cooling rack to set.
Cook's tip: Non-stick baking parchment makes it so much simpler to get these biscuits off the baking trays. You can simply use well-buttered baking trays, but be careful not to leave the florentines for too long or they will harden before you have a chance to lift them off.