Heat the oven to 200C/180C fan/gas 6. Tip the hazelnuts onto a baking sheet, and toast in the oven for 10-15 mins until evenly golden. Tip into a bowl. Line the baking sheet with baking parchment and set aside.
Tip the caster sugar, half the toasted hazelnuts and a small pinch of sea salt into a small saucepan set over a low heat. Cook gently for 7-10 mins until the sugar melts and turns an amber colour – do not stir, but swirl the pan every now and again. Tip the mix onto the baking sheet. Leave to cool completely, then break up into small pieces.
Line the base of a 23cm fluted tart tin with baking parchment. Break the biscuits into a food processor, and blitz with the rest of the hazelnuts to a fine crumb. Or, seal the biscuits and nuts in a food bag and crush with a rolling pin. Tip the crumbs into a bowl and combine with the melted butter. Press into the base of the tin and up the side, then chill for 2 hrs.
Remove the cold tart base from the tin (if it sticks slightly, put it in a warm oven for 3-4 mins to help release it), then set on a serving plate. Spoon the dulche de leche into a bowl and gently fold in a large pinch of sea salt (don’t overmix, or it will become too loose).
Tip the cream, muscovado sugar and vanilla into another bowl and gently beat with an electric whisk until softly whipped. Spread the salted caramel over the base of the tart, then arrange the banana slices on top. Spoon over the muscovado cream. Chill for at least 3 hrs, or overnight. Scatter with the hazelnut praline just before serving.