Preheat oven to 400 degrees. Butter a 12 oz ramekin, then sprinkle with cocoa powder. Shake out the excess and set the ramekin aside for now.
Place chocolate chips and butter in a microwave-safe bowl and microwave for 30 seconds. Stir, then continue to microwave in 15-second increments, stirring etween each heat cycle, until the mixture is melted and well combined. Don't rush this as schocolate can burn very quickly! Set aside for now, allowing it to cool a bit.
Place the brown sugar, egg, egg yolk, vanilla, and salt into a large mixing bowl. Using the whisk attachment of your stand mixer, run on medium speed for about 2 minutes. Stop when the mixture is light and fluffy.
Add the gluten-free flour and whisk on low speed until just combined. Don't overmix.
Add the chocolate mixture to the egg mixture and mix on low speed until just combined.
Pour mixture into the prepared ramekin and bake for 16-18 minutes. Once it's firm on the outside and soft on the inside, pull it from the oven. Be careful not to overbake!
Allow cake to cool for 10 minutes at room temperature. Place the ramekin upside down on a plate, then invert to release the cake onto the plate. Sprinkle with powdered sugar and serve.