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Preheat the oven to 350°. Spray a 12-cup nonstick muffin pan with vegetable oil spray. In a medium bowl, sift the flour and cocoa with the baking powder, baking soda and salt. In another bowl, using an electric mixer, beat the egg yolks with the canola oil, 1/2 cup of the sugar and the water. Beat in the dry ingredients at low speed until smooth.
In a clean bowl, using clean beaters, beat the egg whites at high speed until soft peaks form. Add the remaining 2 tablespoons of sugar and beat until stiff and glossy. Beat one-fourth of the whites into the batter, then fold in the remaining whites until no streaks remain. Spoon the batter into the muffin cups, filling them halfway. Bake for 15 minutes, or until the cupcakes are springy when touched. Let the cupcakes cool for 5 minutes, then turn them out onto a wire rack to cool completely.
In a medium blow, beat the butter with the confectioners' sugar, Marshmallow Fluff and 1 1/2 tablespoons of the heavy cream at medium speed until fluffy. Transfer all but 1/2 cup of the filling to a pastry bag fitted with 1/4-inch plain round tip. Beat the remaining 1 teaspoon of cream into the remaining 1/2 cup of filling and reserve.
Line a large baking sheet with wax paper. Insert the tip of the pastry bag about 1/2 inch deep into the bottom of each cupcake; squeeze lightly to fill with cream. Set the cupcakes on the sheet.
Heat the cream in a small saucepan until steaming. Add the chocolate and let stand for 5 minutes. Add the butter and stir until smooth. Spread the top of each cupcake with the frosting. Spoon the reserved filling into a pastry bag fitted with a very small plain tip and pipe decorative swirls on each cupcake. Refrigerate the cupcakes for at least 10 minutes to set the frosting.