Preheat oven to 350 degrees F and line an 8x8 inch baking pan with parchment paper and set aside.
In a bowl whisk together flour, cocoa powder, and salt, set aside.
Add chocolate, butter, oil, granulated and brown sugar in a heatproof bowl and set over a pan of simmering water, stirring occasionally until melted and smooth, and mixture is glossy, about 5 minutes. The sugars should melt a bit too.
Allow mixture to cool slightly, then add in vanilla and eggs one at a time, whisking in between each addition.
Fold in dry ingredients until just combined.
Pour 3/4 of the brownie batter into the bottom of the prepared pan and smooth into an even layer.
Drop spoonfuls of mint swirl batter onto brownie batter.
Add spoonfuls of the rest of the brownie batter over the mint mixture.
With a small spatula or a table knife, gently swirl the two batters to create a marbled effect.
Bake 30-40 minutes until a toothpick comes out with moist crumbs, but not wet batter.
Allow the brownies to cool completely before cutting into bars.