Preheat oven to 350F and grease an 8x8 baking pan or line with parchment paper
Gently melt butter and chocolate bar together in the microwave or over a double boiler
Add eggs, brown sugar, and vanilla extract to a large mixing bowl or a stand mixer
Mix with the whisk attachment for an electric stand mixer or with an electric hand mixer until the mixture is fluffy and lighter in color about 3 minutes
Fold melted chocolate and butter mixture into the eggs and sugar mixture with a spatula - be sure the chocolate mixture is not too hot
Add cocoa powder and salt to the bowl and fold in with a spatula. Add the chocolate chips and fold in again
Mix with a spatula until you are left with a smooth, even mixture
Pour brownie batter into a baking pan
Bake at 350F for 28 minutesÂ
Allow to cool completely and then slice into squares