Preheat oven to 350°F/180°C. Line a 9x9 baking pan with parchment paper and set aside.
Melt your butter in a small saucepan over medium heat until you begin to get small flecks of brown at the bottom. Remove from heat and let cool.
In a large bowl, whisk the sugars, brown butter, vanilla and salt together. Add in the flour and whisk until fully combined. Press this cookie batter into the pan and bake for 15 minutes. It won't be fully done but it will finish baking when we add the filling. Keep the oven on.
Make the filling
Put the butter, golden syrup, salt, vanilla, brown sugar and double cream in a small saucepan over medium heat. Whisk until the sugar dissolves and then let it come to the boil. Once boiling, take out about ½ cup and slowly pour it into your beaten eggs in a very thin stream, all while constantly whisking the eggs. Once the boiling mixture has been added, slowly stream it back into the saucepan, all while whisking the mixture in the saucepan. Remove from heat and add the pecans and chocolate, then pour over the shortbread.
Bake for 30-35 minutes and let cool completely. I found it easiest to cut them once I had put them in the fridge.