For a shiny, fudgy glaze to finish the cupcakes, we melted semisweet chocolate chips with butter. To finish the look of these cakes, we couldn’t resist piping some curlicues across the top of each cupcake using some extra filling.
1. FOR THE CUPCAKES: Adjust oven rack to lower-middle position and heat oven to 350 degrees. Line 12-cup muffin tin with paper liners.
2. Microwave oil, chocolate, and cocoa together in bowl at 50 percent power, stirring often, until chocolate is melted, about 2 minutes; whisk smooth and let cool slightly. In separate bowl, whisk flour blend, baking powder, baking soda, xanthan gum, and salt together.
3. In large bowl, whisk eggs and vanilla together. Whisk in sugar until well combined. Whisk in cooled chocolate mixture and milk until combined. Whisk in flour blend mixture until batter is thoroughly combined and smooth.
4. Portion batter evenly into prepared muffin tin. Bake until toothpick inserted into center of cupcakes comes out clean, 16 to 18 minutes, rotating muffin tin halfway through baking. Let cupcakes cool in muffin tin for 10 minutes, then transfer to wire rack and let cool completely, about 1 hour. (Unfilled and unglazed cupcakes can be stored in airtight container at room temperature for up to 1 day.)
5. FOR THE FILLING AND GLAZE: Sprinkle gelatin over water in large bowl and let sit until gelatin is softened, about 5 minutes. Microwave until mixture is bubbling around edges and gelatin dissolves, about 30 seconds. Whisk in softened butter, vanilla, and salt until combined. Let mixture cool until just warm to touch, about 5 minutes, then whisk in marshmallow crème until smooth; refrigerate until set, about 30 minutes.
6. Transfer 1/3 cup filling into pastry bag fitted with small plain tip for garnishing finished cupcakes (or zipper-lock bag; trim corner of bag before piping). Reserve remaining mixture for filling cupcakes. Microwave chocolate and 3 tablespoons butter in bowl at 50 percent power, stirring often, until melted, about 30 seconds. Let glaze cool to room temperature, about 10 minutes.
7. Cut cone-shaped piece from center of each cupcake and fill with 1 tablespoon filling. Cut off tip of cone, then replace on cupcakes and press lightly to adhere. Frost each cupcake with 2 teaspoons cooled glaze and let sit for 10 minutes. Pipe curlicues across top of cupcakes with filling in pastry bag, and serve.