Chop Chocolate. To start, make sure all your utensils, bowls, etc. are clean and DRY. Small amounts of water can make chocolate seize and go grainy, which is the opposite of what we're going for here. Then, chop the chocolate into very small shards. The more evenly it's chopped, the more evenly it'll melt.
Melt. In a medium heatproof bowl, combine chopped chocolate and heavy cream. Melt over a double boiler (set the bowl over a saucepan with 1-inch of water simmering over medium heat) or melt in the microwave in 10-15 second increments, until the chocolate is smooth and glossy.
Finish. Stir in butter, vanilla extract and salt, and stir to combine. At this point, the chocolate mixture should be very smooth and glossy, with perhaps a few small air bubbles from stirring. If your chocolate fondue is somehow separated or slightly grainy, you can use an immersion blender to pull it back together.
Serve warm with your favorite gluten-free dippers. (See notes below!)