Preheat the oven to 375 degrees. Butter a small ramekin and set aside.
Melt the chocolate and butter together slowly, stirring carefully.
While the chocolate is melting, whisk together the egg, sugar, salt, and vanilla.
Pour about a quarter of the melted chocolate into the egg mixture and whisk quickly. This will bring the temperature up so we don’t end up with scrambled eggs. Add the rest of the melted chocolate and whisk again.
Stir in the flour until just combined. Scrape every last morsel into the ramekin and bake for 10 minutes. The cake is done when the edges are firm, but the center is still soft. Ovens vary so watch yours like a hawk. Enjoy right away with the toppings of your choice. Some of my favorites are fresh fruit, peanut butter (super rich), whipped cream, jam, and powdered sugar.