Preheat your oven to 350°F (175°C). Generously grease two 6-ounce ramekins with butter, then dust them thoroughly with cocoa powder, tapping out any excess. This prevents sticking and adds a lovely chocolate finish.
In a heatproof bowl set over a saucepan of simmering water (double boiler method), melt the semi-sweet chocolate chips and butter together. Stir occasionally until smooth and fully combined. Remove from heat and let it cool slightly for a few minutes.
In a separate medium bowl, whisk together the egg, egg yolk, granulated sugar, vanilla extract, and salt until pale and slightly thickened, about 1-2 minutes.
Gently fold the slightly cooled chocolate mixture into the egg mixture until just combined. Be careful not to overmix.
Add the gluten-free flour to the chocolate-egg mixture and fold it in until just incorporated. Again, avoid overmixing; a few streaks of flour are okay.
Divide the batter evenly between the two prepared ramekins.
Place the ramekins on a baking sheet and bake for 12-15 minutes. The edges should be set, but the center should still be slightly soft and jiggly. Baking time may vary depending on your oven.
Let the lava cakes rest in the ramekins for about 1 minute. Then, carefully invert each ramekin onto a serving plate. The cakes should slide out easily. If they don't, gently run a thin knife around the edge.
Serve immediately, garnished as desired.