Preheat the oven to 170 degrees Celsius.

Place the softened butter with the brown and white sugar in an electric mixer on high for 3-5 minutes, until pale, fluffy, and creamy.

Add the eggs and vanilla and whisk until the mixture is smooth, with no streaks of egg left.

Add the flour, salt, baking powder, and baking soda, then mix on low speed just until just combined.

Add the chocolate chips and mix into the dough with a spatula.

Line the baking pan with baking paper and press 3/4th of the dough evenly into the base. Place the marshmallows, biscuits, and chocolate bars over the top, then cover with the rest of the dough and add a few extra marshmallows and biscuits. You can bake right away, or cover and chill for up to 24 hours.

Heat the oven to 170 degrees Celsius and bake for 40-45 minutes, until the top is lightly golden. Switch to the broiler for the final 5 minutes so the marshmallows toast and caramelize. Take them out while the centers are still soft, as they’ll keep setting while they cool. Add extra chocolate on top while the bars are hot and let it melt slowly.

Let the bars rest for at least 30 minutes before slicing. They’re best warm, so reheat any leftovers before serving.