Preheat your oven to 350° F. Butter a small (3 to 4 inch) square baking pan or similarly sized oven safe ramekin and set aside.
Place the butter, chopped chocolate and espresso powder in a small microwave safe bowl. Heat in 20 second increments, stirring in between, until fully melted and smooth.
In another small bowl, whisk together the egg, both sugars and vanilla extract until the sugars have dissolved and the mixture has lightened.
Add the chocolate mixture to the egg mixture and stir to combine.
Add the flour, cocoa powder and salt to the mixture and stir just until combined.
If using the mini chocolate chips, gently fold them in.
Scrape the batter into the prepared pan and smooth the top. Place in the pre-heated oven and bake for about 10 minutes. The top should have a glossy finish and the edges should be set.
To serve warm, allow to cool slightly before gently removing from the pan. Otherwise, allow to cool fully before removing from the pan.