Preheat the oven to 350°F (175°C) and prepare an 8x8-inch baking pan.

In a large mixing bowl, combine the all-purpose flour, oats, brown sugar, lemon zest, nutmeg, and salt. Stir until mixed.

Add the cubed cold butter to the dry mixture and cut in until it resembles coarse crumbs.

Pour in the heavy cream and vanilla extract. Stir gently until combined, being careful not to overmix.

Fold in the cinnamon baking chips until evenly distributed.

Press half of the mixture into the bottom of your prepared pan to form an even layer.

Spread the Biscoff cookie spread evenly over the pressed oat mixture.

Place the marshmallows over the Biscoff layer, spacing them evenly.

Sprinkle the remaining oat mixture on top of the marshmallows.

Bake for 25-30 minutes until golden brown and the marshmallows are bubbly.

Allow the bars to cool in the pan for 15 minutes, then transfer to a wire rack to cool completely before slicing.