In a medium bowl, combine granulated sugar, brown sugar, all-purpose flour, cinnamon, and salt. Work in the softened unsalted butter until the mixture resembles coarse crumbs.

Preheat your oven to 350°F (180°C). Line an 8x8-inch baking pan with parchment paper. In a large mixing bowl, beat softened butter and sugars until smooth and creamy. Add the egg, egg yolk, and pure vanilla, mixing until well combined. Gradually incorporate the dry ingredients until a soft dough forms.

Press the cookie dough into the prepared baking pan, filling the corners. Bake in the preheated oven for about 12 minutes, until edges are lightly golden.

Remove the baked cookie base from the oven and let it cool slightly. Spread a generous layer of Biscoff spread over the top. Scatter mini marshmallows across the surface and sprinkle the crumb topping over the marshmallows.

Return the pan to the oven and bake for an additional 12-15 minutes, until the marshmallows are golden and the crumb topping is crisp. Let it cool completely on a wire rack before slicing into squares.