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To prepare the filling: Scoop the Biscoff spread in 1 1/4" balls onto a piece of parchment or plastic wrap. A gently heaped teaspoon cookie scoop works well here. Cover them with plastic, and put them in the freezer for a few hours, or until they're frozen solid.
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Preheat your oven to 350°F. Line a muffin pan with paper muffin cups, and grease the muffin cups.
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To make the batter: Whisk the dry ingredients together in a medium-sized bowl. Whisk the vanilla, vinegar, vegetable oil, and water in a separate bowl. Pour the wet ingredients into the bowl of dry ingredients, stirring until thoroughly combined.
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Divide the batter among the lined wells of the muffin pan, filling each one about 2/3 full.
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Bake the cupcakes for 12 minutes, or until their edges look set but their centers are still liquid.
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To add the filling: Remove the Biscoff balls from the freezer, and quickly place one into the center of each baking cupcake. Don't try to submerge them; just plop them on top. Also, do this with the pan in the oven and just pulled out a bit, if you can; removing the pan from the oven could cause the cupcakes to fall.
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Continue baking the cupcakes for 12 minutes, or until a toothpick inserted into the center of one (avoiding the filling) comes out clean. While the cupcakes are baking, prepare the marshmallow topping.
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To finish the cupcakes: Cut each marshmallow longitudinally nearly in half, leaving a "hinge" on one side. Imagine butterflying a piece of meat — that's what you're after.
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When the cupcakes test done, remove them from the oven. Set your oven to broil.
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Working quickly, criss-cross two strips of marshmallow atop each cupcake, centering them as best you can. Place the pan back into the oven, and broil the cupcakes until the marshmallows are toasted; this could take less than a minute, so keep your eye on them.
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Remove the cupcakes from the oven, and re-center any marshmallows that may have slipped. Let the cupcakes cool to lukewarm, then carefully remove them from the pan and place them on a rack to cool.
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Storage information: Store leftover s'more cupcakes in an airtight container at room temperature for several days. Freeze the baked and filled cupcakes for longer storage; add the marshmallow topping to thawed cupcakes before serving.