Using an electric mixer, whisk the egg yolks and sugar for 2–3 minutes on high speed until pale. Combine the milk, cream and tonka bean in a heavy-based saucepan, bring to the boil over medium heat, then pour in the egg yolk mixture, whisking to combine. Strain into a clean saucepan and cook over low heat, stirring constantly until the mixture coats the back of a spoon. Strain into a bowl placed over an ice bath, then stir occasionally until chilled. Freeze in an ice-cream machine according to the manufacturer’s instructions. Freeze for 6 hours or overnight.
Preheat the oven to 175°C 345°F/Gas 3–4. Mix all the dry ingredients for the chocolate cookies in a large bowl. Incorporate the butter then add the milk and vanilla seeds, stirring until well combined. Turn out the mixture onto a large sheet of plastic wrap and roll into a 5 cm (2 inch) diameter log. Wrap tightly and refrigerate for 2 hours. Thinly slice into 3 cm (1¼ inch) slices. Lay the slices on a large baking tray lined with baking paper and bake for about 10–15 minutes, or until just cooked through.
To make the hazelnut chocolate cream, put the sugar in a saucepan with 1 tablespoon of water and place over medium heat. Heat until the sugar dissolves, bring to the boil and cook for 4–5 minutes until caramelised. Add the cream (be careful, hot caramel will spit) and whisk to combine. Working quickly, add the hazelnuts, then pour the mixture into a blender and blend until smooth. Pour the warm mixture over the chopped dark chocolate in a bowl, and stir until the chocolate has melted and the ingredients are thoroughly combined.
To make the brittle, brush a baking tray with vegetable oil and set aside. Combine the sugar, butter, rice syrup and 75 ml (2¼ fl oz) of water in a saucepan over high heat and cook for 20–30 minutes until golden and caramelised, stirring occasionally to prevent the caramel from sticking to the base. Add the hazelnuts, stir to combine and pour onto the prepared tray. Set aside in a cool, dark place for 1 hour to cool and set. When set, break into pieces and set aside until serving. Brittle will keep in an airtight container in a cool, dry, dark place for 2–3 days.