In a microwave-safe mug or ramekin, add the chopped dark chocolate and butter. Microwave in 20-second intervals, stirring each time, until melted and smooth.

Whisk in the sugar, egg, and vanilla extract until fully combined.

Gently stir in the flour and salt until just mixed. Do not overmix.

If using, stir in 1 tablespoon milk or heavy cream for an even silkier texture.

Microwave on medium-high (70-80% power) for 45-60 seconds. The edges should be set, but the center should still be slightly jiggly. If needed, microwave in 5-second increments, checking for done-ness.

Let sit for 30 seconds before serving.