Financiers are so-called because of their shape – little sponges baked in a mould to look like gold bars. To add to the effect, Prue has finished these with tiny pieces caramel, like extra nuggets of gold.
Heat the oven to 230°C/210°C fan/450°F/Gas 8.
First, make a brown butter for the financiers. Melt the butter in a pan over a medium heat. Once melted, increase the heat and boil gently until the butter has turned golden brown and there is a ring of browned butter around the edge of the pan. Do not allow the butter to burn. Take care as it will spit and splatter a little. Remove the pan from the heat and pass the butter through a sieve lined with a sheet of kitchen paper. Leave any brown residue stuck to the bottom of the pan – don’t scrape it out.
Tip the walnuts into the bowl of a food processor and blitz until ground. Do not over process. Add the ground almonds, caster sugar and flour to the bowl and pulse a few times until mixed with the walnuts. Do not over process.
Tip the mixture into a large bowl and make a well in the centre. Pour the egg whites into the well and whisk gently for 1 minute, until smooth, then whisk in the warm brown butter until you have a smooth, glossy batter. Pour the mixture into a jug, then divide it equally between the 12 moulds.
Bake the financiers for 5 minutes, then reduce the oven temperature to 200°C/180°C fan/400°F/Gas 6 and bake for a further 10 minutes, until risen and a skewer inserted into the centre comes out clean. Remove from the oven and leave to cool in the mould for a few minutes, then remove the financiers from the moulds and transfer them to a wire rack to cool completely.
For the caramel, tip the sugar into a heavy-based frying pan. Heat over a high heat to dissolve the sugar, swirling the pan from time to time, but do not stir. Once the sugar turns a deep caramel, carefully pour it out onto a silicone mat and leave it to cool and harden.
Make the buttercream. Combine the butter, salt, vinegar, vanilla and espresso powder in the bowl of a stand mixer fitted with the beater attachment, then beat on medium speed until smooth.
Add half the icing sugar and half the cream, then beat on low speed for 3 minutes, until very smooth. Add the remaining icing sugar and cream and beat for a further 3 minutes, scraping down the sides of the bowl as necessary, until the mixture is fully incorporated. Spoon the buttercream into the piping bag.
Using a large sharp knife, very carefully chop the set caramel into small pieces.
When the financiers are cool, turn them all with a narrow end closest to you. Starting at the end farthest from you, pipe the icing from side to side across the top of the financiers. Sprinkle with the caramel, and sprinkle with gold edible powder (if using).