Toast the grind fennel seeds. In a small skillet over low heat, lightly toast the fennel seeds. Once fragrant and lightly browned, add to a pestle and mortar and grind to a powder. Set aside.
Heat the sugar until it melts. In a high-sided nonstick saucepan, heat the sugar over medium heat, stirring often. Once the sugar is in liquid form, stop stirring and just swirl the liquid sugar whilst the colour changes to a lovely amber colour. Keep an eagle eye on it at this stage as it can turn to burnt sugar very quickly.
Add the butter. Carefully add the butter in one go. It will bubble up but whisk continuously until it has thoroughly melted. Then remove from the heat.
Add the cream. Pour in the cream, but be careful as it will bubble up again, but keep whisking and the bubbling will die down. Return it to medium heat, stirring occasionally, so that the caramel can thicken. This takes around 5 minutes.
Add the flavourings. Remove from the heat, stir in the ground fennel seeds (add half if you want a more subtle flavour), vanilla extract and salt, and stir thoroughly to combine. Pour into a bowl and set aside to cool for 10 minutes.
Add to the tart shell. Pour 1 cup (240 ml) into the bottom of the tart shell and smooth out flat. Pour any remaining caramel into a small serving dish and serve alongside the tart.