On a cutting board, chop the chocolate into small pieces.
In a glass, liquid measuring cup melt the chocolate in the microwave. 30 seconds at a time and stir. Melt the butter with the chocolate. Let cool and set aside.
In a bowl, whisk the eggs and vanilla. Incorporate the brown sugar and sugar, whisk together well. Add the melted, cooled chocolate and butter mixture. Whisk in to incorporate. Add the oil and gently whisk in. Add the flour and salt and gently whisk in.
Fold in any optional ingredients, such as chocolate chips, nuts or Reese's peanut butter cup pieces.
Spray a 9x9-inch pan with cooking spray or brush with shortening. Pour the brownie batter into the prepared pan.
Bake in a preheated 350 degree F oven for 40-45 minutes or until a toothpick when inserted comes out clean. Let cool completely before cutting into squares.