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1
Preheat the oven to 375 degrees F. Line 2 baking sheets with parchment paper.
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2
For the molasses cookies: Put the brown sugar, butter and vegetable shortening in a large bowl and beat with a hand mixer on medium speed until combined.
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3
Add the flour, molasses, eggs, cinnamon, ginger and baking soda and beat until combined.
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4
Use a tablespoon to scoop the dough onto the baking sheets.
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5
Make sure there is ample space between each dough ball as the cookies will spread.
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6
Bake until puffed on top, 10 to 12 minutes.
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7
Cool the cookies on the baking sheet on a rack for 5 minutes, then transfer the cookies to the rack to cool completely before frosting.
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8
For the boiled frosting: Combine the granulated sugar, 3/4 cup water, cream of tartar and a pinch of salt in a medium heavy-bottomed pot, bring to a boil over medium heat and cook until a candy thermometer registers 245 degrees, about 3 minutes.
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9
Remove from the heat and fold in the marshmallows until melted.
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10
Put the egg whites in the bowl of a stand mixer fitted with a whisk attachment and whip on medium-high speed into stiff peaks.
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11
Turn the mixer to low and slowly pour the hot sugar mixture into the egg whites.
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12
Beat on medium speed until cool and fluffy, then fold in the vanilla extract.
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13
Using a butter knife, frost the bottoms of half the cookies, top them with the remaining cookies, and serve.
