Grease and line a deep 20cm x 20cm tin with baking parchment
Make the caramel: put the condensed milk, butter, sugar and syrup into a heavy based pan and melt, over a medium heat, stirring frequently,until the sugar has dissolved, then bring up to boiling
Allow the mixture to gentle boil for 2-3 minutes. Take off the heat, stir in the sea salt and set aside
Put 20g butter into a small, heavy based saucepan. Heat on low-medium, until melted. Add 110g marshmallows and stir with a wooden spoon, until the marshmallows have melted and are thoroughly blended with the butter. Quickly mix in half of the rice krispies
Spoon the marshmallow krispies into the tin, and spread out evenly. Ensure the mixture is pressed down gently but firmly to create a solid layer. The mixture will be very sticky but using a piece of cellophane makes this task much easier.
Give the caramel a stir and pour over the krispie base. Scatter the chopped pecan nuts over the top and place in the fridge for 1-2 hours to set
Using the remaining butter, marshmallows and rice cereal, make another batch of marshmallow krispies and spoon it over the caramel layer. Again, using a piece of cellophane, spread the krispies out and press down gently, but firmly
Leave to firm up for an hour before slicing into squares
Store in an airtight tin. Best eaten within 2 days