Preheat the oven to 180°C.
Melt the chocolate in a heatproof bowl over a pan of barely simmering water. Once melted, set to one side to cool slightly.
In a separate bowl, beat together the butter and the sugar until light and fluffy.
Fold in the melted chocolate, the ground almonds, egg yolks and peppermint oil until smooth.
Whisk the egg whites with a pinch of salt until they form soft peaks and then fold them into the chocolate mixture using a metal spoon. Carefully fold in the sifted flour making sure not to knock out too much air. Spoon into a greased 9 springform inch cake tin and bake in the oven for 40 mins until it feels springy to the touch.
Meanwhile, whisk together the double cream and the peppermint oil with the icing sugar until it forms soft peaks and refrigerate.
For the ganache topping, heat the double cream and peppermint oil until boiling point and pour over the chopped dark chocolate. Stir until smooth and leave to cool and thicken a little.
Remove the gateau from the oven and let it stand for 5 minutes before removing from the cake tin and cooling on a wire rack.
Once cool, cut the cake horizontally in half. Spread the bottom half with the whipped cream and place the other half on top. Pour over the ganache and leave to cool and set.
Dust 10 of the mint leaves with cocoa powder and arrange on top of the cake along with the green mint leaves.
*This pudding is very rich and indulgent, so shouldn't be eaten too often. But it's perfect for those special occasions!