Preheat oven to 350°F (175°C).

Melt butter in an 8-inch oven-safe skillet over low heat on the stovetop.

Remove skillet from heat and whisk in granulated sugar until well combined.

Add the egg and vanilla extract, whisking vigorously until the batter is glossy.

Sift in cocoa powder, all-purpose flour, and salt, then stir with a spatula until just incorporated.

Fold in half of the semi-sweet chocolate chips into the batter.

Smooth the surface and sprinkle the remaining chocolate chips on top.

Transfer the skillet to the oven and bake for 25 to 30 minutes until the edges are firm.

Remove from oven and let cool for at least 10 minutes before slicing and serving.