Preheat oven to 350F degrees.
In a small pot, melt together butter and chocolate on low heat. Stir constantly. Once fully melted, remove from heat and let cool for a few minutes.
Meanwhile, using a stand mixer or a hand mixer, add eggs, sugar, and vanilla extract to a medium mixing bowl. Beat on high for about 5 minutes until mixture becomes pale white and fluffs up a bit.
Slowly add the chocolate mixture to the bowl with the egg and sugar mixture while mixing at the same time. Mix until fully incorporated.
Add in the flour and sea salt and mix one last time until just incorporated.
Pour into a greased and lined 8x8 baking pan. Bake for 40-45 minutes. I cook mine closer to 40 minutes for extra fudgy. Remove from oven, let cool, and enjoy!
Cut into 9-16 pieces depending on what you prefer! Best cut with a plastic knife or wet your sharp knife for clean lines!