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Heat oven to 350 degrees.
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In a large bowl, cream together the butter and sugar with a hand mixer...about 4 minutes.
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Add the egg and vanilla.
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In a separate bowl, mix together the dry ingredients: flour, cocoa, baking soda, and salt.
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Pour some of it into the butter/sugar mixture, alternating with the buttermilk..dry mix, buttermilk, dry mix, buttermilk, etc.
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Drop the batter onto sheet pans covered in parchment paper. Give the cookies plenty of room to spread.
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Bake for about 7 minutes, rotate the pan in the oven and bake for 7 minutes more OR until the cookies "spring bag" when lightly pressed with your finger.
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For the filling, beat the softened butter and sugar until fluffy.
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Add the vanilla and salt.
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Beat in the marshmallow creme. Mix well.
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Refrigerate for about 30 minutes.
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Dollop on a cookie, take another cookie on top and press together until the filling spreads to the edges.