Whisk together the flour, cocoa powder, salt, and chocolate chips in a medium bowl until combined.

Melt the butter, then whisk in both sugars until glossy. Add the eggs and vanilla and whisk until thick and shiny. Fold the dry ingredients into the wet mixture just until no streaks of flour remain.

Scrape the batter into a greased 6-inch microwave-safe dish, smooth the top, and microwave on high for 1 minute 30 seconds. Check for doneness, then continue in 15-second bursts until the center is just set and the edges pull away slightly.

Cool in the dish for at least 5 minutes, then slice into squares and serve.