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1
Heat the oven to 350F Line an 8-inch-square baking dish with foil, leaving about 1 inch hanging over 2 edges.
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2
Lightly coat the bottom and sides with cooking spray.
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3
Make room in the fridge.
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4
Put 3 Tbs.
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5
of the butter in a medium, heatproof bowl and heat in a microwave or over simmering water until melted.
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6
Add the cookie crumbs and stir until the crumbs are moist and well blended.
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7
Dump the crumbs into the prepared pan and, using a flat-bottomed cup, firmly press the crumbs into an even layer on the bottom.
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8
Bake until fragrant, about 12 minutes.
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9
Set on a rack to cool.
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10
Put the chocolate and the remaining 5 tablespoons butter in a medium heatproof bowl and heat in a microwave or over simmering water until melted.
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11
Whisk until smooth and blended.
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12
Pour into the baked crust (it's OK if the crust is still warm) and spread evenly.
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13
Refrigerate until almost firm, about 30 minutes (for faster chilling, slide the pan into the freezer).
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14
Arrange the marshmallow halves, cut side down, evenly over the chocolate, pressing lightly.
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15
Cover and refrigerate until very firm, about 40 minutes or for up to 2 days.
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16
Just before serving, adjust the oven rack to the top level and heat the broiler on high.
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17
Using the excess foil as handles, lift the s'more square from the pan and set on a cookie sheet.
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18
Pull the foil away from the sides and slide the cookie sheet under the broiler.
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19
Broil until the marshmallows are browned, about 20 seconds.
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20
Set on a rack and move the foil and s'more square to a cutting board.
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21
Cut into 18 rectangles.
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22
Serve immediately or cover and refrigerate for up to 2 days.
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23
Serve slightly chilled.
