For chocolate cupcakes:

1. Pre-heat the oven to 355°F (180°C) and line a cupcake tin with liners.
2. In a small bowl, mix together the eggs, yogurt or sour cream, milk, and lemon juice. Set aside.
3. Sift together the gluten free flour blend, cocoa powder, xanthan gum, and baking powder. Add the sugar and salt and mix well.
4. Add the softened butter to the dry ingredients. Using a stand mixer with the paddle attachment or a hand mixer, beat until pea-sized pieces form.
5. Add the milk-and-egg mixture and whisk until you get a smooth batter.
6. Add the melted chocolate and whisk until evenly incorporated.
7. Divide the batter evenly among the cupcake cases, filling each about 3/4 full.
8. Bake at 355°F (180°C) for about 20 minutes, or until a toothpick comes out clean. Allow to cool completely.

For chocolate fudge sauce:

1. Place the chocolate in a heat-proof bowl. Heat the coconut cream and golden syrup in a small saucepan over medium heat until it just begins to simmer.
2. Pour over the chocolate and let sit for 1 minute, then stir from the center outward until the sauce is smooth and glossy.
3. Allow to cool until thick enough to pipe or spoon, but still soft. (Refrigerate briefly if needed.)

For chocolate buttercream frosting:

1. Beat the butter with a stand or hand mixer for 2–3 minutes until pale and fluffy.
2. Add the powdered sugar and beat for a further 5 minutes.
3. Add the cocoa powder and salt and beat until evenly distributed.
4. Add the melted and cooled dark chocolate and beat until you have a rich, fluffy, evenly colored frosting.

Assembling the cupcakes:

1. Use an apple corer or a piping nozzle turned upside-down to create a hole in the center of each cooled cupcake. Fill with the chocolate fudge sauce.
2. Pipe the chocolate buttercream frosting on top using a piping nozzle of your choice.
3. Sprinkle with chocolate shavings or sprinkles. The cupcakes keep well in a closed container in a cool dry place for 2–3 days.