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Preheat oven to 350 degrees.
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Grease a cookie sheet, set aside.
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In a small bowl, combine flour, baking soda, salt, and baking powder; set aside.
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In a large bowl, beat butter and peanut butter with an electric mixer on medium speed until combined.
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Beat in granulated sugar and brown sugar until fluffy.
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Beat in egg and vanilla.
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Using a wooden spoon, stir in the flour mixture and oats just until combined.
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Drop dough by rounded teaspoons 2 inches apart on prepared cookie sheet.
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Bake 8-10 minutes or until edges are light brown and centers are set.
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Cool on cookie sheet for a few minutes, then transfer them to a wire rack.
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In a medium bowl combine the hot water and the salt, stirring until salt dissolves.
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Add marshmallow creme, shortening, and powdered sugar.
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Beat with an electric mixer on medium speed until combined.
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Spread marshmallow mixture on the bottom sides of half the cookies. Top with the remaining cookies, bottom sides down, to make cookie sandwiches.