Melt butter, condensed milk, and chocolate.
Whisk together eggs, egg yolk, sugars, and vanilla on high.
After 2 minutes of mixing the mixture will look airy and bubbly like this!
Fold in flour and unsweetened cocoa powder.
Make sure not to overmix!
Pour into prepared 8x8 pan and top with dark chocolate chips.
Bake at 350°F. Cool before drizzling the hot fudge sauce. Set & enjoy!