1 Prep the pan: Lightly butter an 8-inch loaf pan and line with a sling of parchment paper, leaving a few inches of overhang on two sides. The butter will help the parchment paper stick to the pan without slipping.
2 Make the chocolate: Combine the chocolate, cream, and a pinch of salt in a heatproof bowl and set it over a pan of simmering water. Make sure the water does not touch the bottom of the bowl. Stir the mixture over the heat until just melted and smooth.
Remove from the heat and whisk in the champagne.
3 Chill the chocolate block: Scrape the chocolate into the prepared loaf pan with a spatula and smooth the surface. Chill in the refrigerator for at least 8 hours or up to 24 hours, until set firmly (there will still be some slight give, which is good).
4 Slice the truffles: Warm a knife in hot water, dry it off, and then run it around the rim of the pan to loosen the chocolate block. Lift out the block using the parchment sling.
Trim the ends of the truffle to square them off if you want. (And save the trimmings for yourself!) Cut the block in half through the middle to make two squares. Cut each square into thirds and then cut into fourth to make cubes. As needed to prevent sticking, wipe down your knife, dip it again in warm water, and wipe dry.
5 Coat the truffles with cocoa. Place the sifted cocoa powder in a bowl. Carefully toss each cube cocoa powder. If you want to apply gold leaf, make sure to leave one of the sides clear so the leaf has something to stick to.
6 Apply the gold leaf (optional): Avoid touching the gold leaf with your fingers at it will stick to whatever it touches. Use a very dry, very clean paint brush to apply it to the truffles. Lightly brush or dab the surface of the gold leaf with your paint brush to “grab” some of the gold leaf, and then dab it on the surface of the truffle. The leaf should adhere to the truffle and slip off of the brush.
Repeat until all the truffles have a little gold leaf on top.
7 Serving and storing: If serving the same day, arrange the truffles on a plate and store in the fridge until serving. Take them out of the fridge a little bit before serving to take the chill off. Truffles can also be stored in an airtight container for up to 3 days.