Preheat oven to 350°F degrees.
In a large bowl beat butter & brown sugar until combined.
Add egg & vanilla & mix until just combined.
Slowly add flour, cocoa, baking soda, & salt - mix.
Fold in both types of chocolate chips
Scoop batter into a greased or well-seasoned 10-inch ovenproof skillet (cast iron).
Smooth the top by pressing the dough with wet fingers.
Bake until just set in center and pulling away from sides, approx 20 to 22 minutes (it's okay if the center looks slightly underbaked).
Immediately after removing from the oven, sprinkle with additional chocolate chips
Serve warm right from the skillet, with ice cream & chocolate syrup