Preheat the oven to 350F and line two baking sheets with parchment paper
Make the chocolate filling: in a saucepan over low heat, melt together the dark chocolate, heavy cream, and butter until fully melted. Pour into a bowl and allow to cool until just warm. Add the sugar, egg, vanilla, and salt. Mix gently until smooth and glossy and set aside
Make the cookie dough: cream together the softened butter and light brown sugar until pale and fluffy. Add the egg yolks and vanilla, and mix well to combine
In a separate bowl, whisk together the flour blend, cocoa powder, xanthan gum, baking powder, and salt. Add to the wet ingredients and mix well until you get a smooth cookie dough with no flour clumps
Pour the granulated sugar in a bowl. Use a 2 tbsp scoop (or weigh out about 1/18 or 1/19 of the dough) to scoop cookie dough. Roll into a ball, then coat in granulated sugar and place on baking tray at least 2 inches apart
Use a 1 tbsp measuring spoon to make an indent in the cookies
If the chocolate filling has firmed up too much, reheat gently by placing the bowl into a larger bowl filled with hot water and stir until loosened (don't microwave!)
Fill the cookies with about 1 tbsp of chocolate filling - it'll be full to the brim, but it won't overflow!
Bake the cookies 1 sheet at a time for about 9-10 minutes. They should spread and puff slightly, and the filling will still be wobbly in the middle when you shaking the baking sheet
Let cool on the baking sheet until slightly warm - they are fragile until cooled
Make the Swiss meringue: combine the egg whites, sugar, and cream of tartar in a heat-proof bowl above a pot of simmering water until it reaches 160F, whisking constantly. Remove from heat and transfer to a stand mixer fitted with the whisk attachment and whisk for 5-7 minutes on high speed until stiff peaks form and mixture has cooled down. Add vanilla and stir to combine
Transfer Swiss meringue to a piping bag fitted with whatever fun piping tip you'd like to use and make some swirls, stars, rosettes, or any other fun shapes on your cookies
Using a kitchen torch, torch the meringue until toasted for a perfect toasted-marshmallow flavor for your cookies