Beat egg whites lightly in a large bowl using an electric mixer until frothy. Slowly beat enough confectioners' sugar into egg whites to make icing that will hold shape; blend in lemon juice until desired consistency is reached.
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Arrange cookies about 1-inch apart on a large sheet of waxed paper. Spread icing onto each cookie, allowing icing to drip over edges to look like melting snow.
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Grease a microwave-safe plate. Place 1 marshmallow onto plate.
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Microwave marshmallow until it starts to puff up, 20 to 30 seconds. Immediately place marshmallow on the edge of 1 cookie while the icing is still wet to look like a melted snowman. Repeat with remaining marshmallows and iced cookies.
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Decorate 'melted snowmen' by piping 'twig-like arms' using black decorating gel on the icing. Make 'coal eyes' and a 'mouth' on the marshmallow using black decorating gel. Make a 'carrot nose' under the 'coal eyes' using orange decorating gel. Make 'coat buttons' by arranging silver nonpareils down the middle of the icing. Make 'scarves' or 'bow-ties' using red decorating gel or green decorating gel.
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Allow cookies to fully dry before removing from waxed paper, at least 1 hour.
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