Cube the butter and melt it in a small saucepan, over a medium to high heat. Once melted, you want to keep heating it, stirring occasionally with a wooden spoon, until brown specks appear and you can feel that nutty aroma. It will become foamy, that's fine. Be careful not to burn the butter, but also don’t “underbrown” it - it needs to be golden brown. Once it's browned, transfer it to the bowl of a standing mixer and let it cool a bit, so it doesn't cook the eggs.
Preheat your fan oven to 160˚C (325˚F) or 175˚C (350˚F) if you're not using a fan oven. Grease and line with baking paper an 8×8 inch (20×20 cm) square cake pan. Make sure all the sides are covered with baking paper, as well.
Using a food processor grind the biscoff cookies. In a separate bowl, combine them with melted butter. Press the mixture into the bottom of the pan. Bake for 5-6 minutes then set aside to cool.
In a medium sized bowl, sift and combine flour, baking powder and salt. Set aside.
In the bowl of your standing mixer, fitted with the whisk attachment, combine brown butter and both of the sugars. Mix on medium-high heat until just combined, approx. 30-45 sec.
Mash the bananas using a fork and add to butter/sugar mixture, along with two large eggs and vanilla extract. Mix until everything is incorporated.
Lastly, add in your dry ingredients and mix on low speed until just combined.
Pour the batter into the prepared cake pan with cookie crust. Bake in a preheated oven for approx. 35 minutes.
It's done when you stick a skewer in the middle and it comes out with a few moist crumbs (not underbaked batter). Let it cool in the pan on a wire rack before covering with chocolate and marshmallows.
Place finely chopped dark chocolate in a medium sized bowl and pour over it almost boiling heavy cream. Cover it with a plate and leave it for 2-3 minutes. Stir with a silicone spatula to combine.Pour over your cooled blondie. Leave to set a little bit, then cover with marshmallows.
Place the blondie in the freezer for 10-15 minutes, so it's easier to remove it from the pan and you get clean sides.Place it on a serving plate, a wooden board or a wire rack and using a blow torch brown the marshmallows.
Cut into 16 smaller or 9 big pieces and enjoy! :)
If you don’t have a stand mixer, you can use an electric hand mixer.
If you still don’t own one, I advise you to buy a kitchen scale – they’re cheap and easy to use, and always guarantee the same results in baking!
Not all ovens are the same, check the blondie at 30 minute mark, then adjust the time accordingly. Don’t overbake it!
If you don't have a blow torch, you can broil the blondie in the oven using a broil/grill option.