Preheat Oven: Set your oven to 350°F (175°C).

Melt Chocolate and Butter: In a pan or using the microwave in small increments, melt the chocolate chips and butter together, stirring constantly until smooth.

Prepare Wet Ingredients: In a small bowl, whisk together the egg, egg yolk, sugar, vanilla, and salt until well combined.

Combine Ingredients: Add the flour to the wet ingredients and whisk to reduce clumps. Gently fold in the melted chocolate mixture until fully incorporated.

Pour Batter: Pour the batter into a greased, oven-safe dish. A 4 ½-inch-wide ramekin works well. You can also divide the batter into two smaller dishes, but note that the baking time will be shorter.

Bake: Place in the oven and bake for 14-20 minutes, or until the top is just set. Be careful not to overbake.