Preheat the oven to 200C/180C Fan/Gas 6.
For the chocolate shortcrust pastry, sift the flour, icing sugar and cocoa powder into a bowl. Rub in the butter using your fingertips then add the egg yolks, lemon juice and 2 tablespoons cold water. Bring the dough together using your hands (if the dough is too dry add a splash more water). Wrap the dough in cling film and chill for 5 minutes in the freezer, or 15 minutes in the fridge.
On a lightly floured work surface, roll out the pastry to the thickness of a £1 coin and use it to line a 23cm/9in round loose-based flan tin, or a large rectangular tart tin. Prick the base, line with baking parchment and fill with baking beans.
Place the tin on a baking tray and bake for 25 minutes. Remove the baking paper and beans and bake for a further 10 minutes. Set aside to cool.
For the chocolate ganache, bring the cream to a boil in a saucepan. Put the chocolate in a heatproof bowl. Remove the cream from the heat and pour over the chocolate. Stir until the chocolate is melted and incorporated into the cream. Add the butter and stir to combine. Reserve a ramekin of the ganache to make the truffles and place it in the fridge. Divide the remainder between 2 bowls and keep at room temperature.
For the salted caramel, warm the sugar, butter and golden syrup in a pan until the sugar is dissolved. Add the cream and bring to a simmer. Transfer to a bowl and leave to cool completely. Once cool, stir in the peanuts then pour into the cooled pastry case and chill in the fridge until set.
For the cinnamon peanut butter, put the peanuts in a food processor and pulse until the peanuts come together in a ball. Add the oil, honey and cinnamon and process to a smooth paste.
For the chocolate peanut butter mousse, whip the cream with the icing sugar and cornflour until soft peaks form when the whisk is removed. Fold in 1 bowl of the reserved ganache and 2½ tablespoons of the cinnamon peanut butter. Spread the mousse over the chilled tart when the caramel has set.
For the chocolate decoration, melt the chocolate in short bursts until reaches 32C on a kitchen thermometer. Spoon into a disposable piping bag and line a baking tray with non-stick baking paper. Snip off a tiny corner of the piping bag and pipe decorative shapes onto the paper. Set aside to cool and set.
For the peanut powder, if using, add the peanut butter and tapioca maltodextrin to a food processor and pulse until combined. Shape the ramekin of chilled ganache into 3 balls and coat them in the peanut powder to make truffles.
To assemble the tart, when the mousse layer is set, spread a layer of the ganache over the top. Return the tart to the fridge until the ganache is set.
Remove the tart from the tin and dust the top with cocoa powder. Decorate with the truffles and chocolate shapes.