To start, make the short crust pastry base. In a large bowl mix together the butter and sugar with your hands, and finally add the egg yolks and mix.
Add the flour and beat until a soft dough is formed. Gather the dough together and press it into the bottom of the tin and 2cm up the sides (I rolled my pastry).
Bake for about 18-20 minutes, or until golden (I blind baked my pastry with baking beans).
Remove from the oven, but leave the oven on whilst you make the filling.
To make the filling; in a medium saucepan combine all the ingredients except the nuts and vanilla extract. Bring to the boil, stirring until the butter melts and the mixture is smooth. Boil for 30 seconds.
Remove from the heat, then mix the nuts and vanilla extract.
Pour the hot filling over the crust and bake in the oven for about 15 minutes until the filling is bubbling in the centre.
Allow to completely cool in the tin where the filling will become firm, then chill in the refrigerator for at least 1 hour and up to 2 hours.
When chilled, cut into bars with a large sharp knife.