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1
Preheat the oven to 350u00b0F. Line a 6- and 12-muffin tin with paper liners.
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2
Make the cupcake batter: In the bowl of a heavy-duty stand mixer, cream the 2 sticks of butter and the granulated sugar together at medium speed just until light and fluffy, about 1 minute. Add the eggs, one at a time, and mix just until combined. In a measuring cup, stir 1/2 the cup of hot water and the cocoa together until smooth. Add the cocoa mixture to the butter mixture and mix on low speed for an additional 10 seconds.
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3
In a medium bowl, whisk together the flour, baking soda, and salt. With teh mixer on low speed, gradually add the flour mixture to the butter mixture in batches alternating with the milk, beginning and ending with the flour and beating after each addition until the ingredients are just blended.
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4
Fill each prepared muffin cup half full with chocolate batter. Bake for 22 to 25 minutes, or until a toothpick inserted in the middle comes out clean. Let cool completely on a wire rack.
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5
Make the filling: Using a clean bowl and a stand mixer or an electric hand mixer, beat the marshmallow fluff, the 1/3 cup butter, and the 1/3 cup of powdered sugar together until combined and fluffy, about 1 minute. Using the handle of a small fork or spoon, make a hole in the top center of each cake. Gently rotate the utensil in each hole to create a small cavity at each opening. Transfer the filling to a piping bag and pipe in just enough marshmallow mixture to fill each hole. As with the Twinkie, you want a firm grasp on your cupcake as you infuse it with the filling, so as not to allow it to explode; once you feel it growing in size, stop infusing. Use a wet fingertip or the back of a spoon to tamp down any marshmallow peaks, ensuring that the filling is even with the top of the cupcake.
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6
Make the frosting: In a small saucepan, heat the cream over medium heat until bubbles form at the edges. Add the chocolate morsels and remove the pan from the heat, stirring until the chocolate melts. Add the 1 1/2 tablespoons of butter and continue to stir until smooth. Let cool for three minutes. Transfer the chocolate to a large deep glass bowl. Dip the top of each cupcake into the chocolate to coat, letting the excess chocolate drip back into the bowl. Let the cupcakes rest on a wire rack set over newspapers until the chocolate is set, about 30 minutes.
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7
Make the frosting decoration: In a small bowl, beat the 1/4 cup butter and the 1/2 cup of powdered sugar together until smooth, about two minutes. Transfer the frosting to a piping bag and decorate the top of each cupcake with a curlicue pattern. Serve immediately. Store in an airtight container for up to 2 days.
