Preheat the oven to 325°F (160°C). Line an 8-inch (20 cm) square baking pan with parchment paper (if needed, grease the bottom slightly so the paper can stick to the pan). Make sure the paper rise up to also line the sides of the pan so that you can lift the baked brownies out of the pan easily.
Then, chop the chocolate bar into chunks. In a medium bowl, combine half of the chocolate and the oil together and heat in the microwave in 30-second intervals or until melted, about 1 to 1 ½ minutes. Let the mixture sit for about 3 minutes and stir well until smooth. Set aside!
In a separate large bowl, mix the sugar, sifted cocoa powder, and salt, and then add the eggs. Beat with an electric hand mixer for about 3-4 minutes to incorporate air. Whisk in the vanilla and the melted chocolate (make sure it’s not too hot or else the eggs will cook) by hand.
Sift in the flour and fold it in with a spatula to incorporate it into the batter, but do NOT overmix; otherwise, you will wind up with cakey brownies.
Reserve about 1/3 cup of the remaining chocolate chunks and fold the rest into the batter. Transfer the thick brownie batter to the prepared baking pan and level with a spatula. Place the reserved chocolate chunks on top.
Bake for 40-45 minutes or until the edges start to pull away from the pan, the top is set and the center (if gently touched) is still slightly soft but not jiggly. Let the pan cool completely on a rack for about 40-60 minutes before lifting up the parchment paper and slicing our easy homemade brownies into 16 slices.