01 - Preheat the oven to 175°C. Line a 20x20 cm baking pan with parchment paper, allowing an overhang for easy removal.
02 - In a mixing bowl, combine rolled oats, flour, light brown sugar, and fine sea salt. Stir in the melted butter and vanilla extract until the mixture forms coarse crumbs.
03 - Press the oat mixture evenly into the prepared pan. Bake for 15 to 17 minutes, or until lightly golden. Let cool for 10 minutes.
04 - In a separate bowl, blend pistachio butter, powdered sugar, softened butter, vanilla extract, and a pinch of salt until smooth and creamy.
05 - Evenly spread the pistachio butter filling over the cooled oat biscuit base.
06 - Scatter the mini marshmallows evenly over the pistachio butter layer.
07 - Using a kitchen torch, toast the marshmallows until golden and lightly charred. Alternatively, place under a broiler for 1–2 minutes, watching carefully.
08 - Allow the assembled bars to cool completely. Once set, slice into portions and serve.