Melt your butter in a medium sized bowl in the microwave for 20-30 seconds until melted. Remove and mix in the vegetable oil and then whisk in the sugar. Make sure the mixture is not too hot. If it is, wait a few minutes.
Add in the eggs and vanilla and continue whisking. In a microwave safe glass melt mini chocolate chips in 30 second intervals until melted - mixing in between microwaving. Let cool a couple minutes, then whisk into your batter.
Add the flour, cocoa powder, baking soda, and salt into the batter. Switch to a rubber spatula. Fold the dry ingredients into the batter until just combined - do not overmix. Now fold in the regular sized chocolate chips. Preheat your oven to 350F. Prepare your 8x8 inch square pan. I like to butter it, then line the pan with parchment paper.
Pour the batter in and smooth it out with the rubber spatula. Bake 30-35 minutes, until the top is crinkly, but it is no longer wiggly in the middle when you move the pan. You do want some movement, so that it can be soft and fudgy. Remove and let cool completely before cutting into squares and eating. Enjoy!