Preheat oven and place 12-inch cast iron skillet inside.
Beat eggs, sugars, and maple syrup for 10 minutes.
Melt butter and chocolate; whisk in cocoa and vanilla.
Combine chocolate mixture with egg mixture.
Mix in sourdough discard.
Add flour and salt; mix gently.
Grease hot skillet and pour in batter.
Bake 30–40 minutes until edges set and center is fudgy.
Cool at least 30 minutes before slicing.