Prep the ice cream:

1. Freeze. Line a 9x13-inch baking pan with parchment paper. Transfer the ice cream into the baking pan and use a spatula to spread it out until smooth and flat (this may take several minutes). Transfer the ice cream into the freezer and freeze for at least 4 hours.

Make the cookie layers:

1. Preheat. Preheat the oven to 350F. Prepare a nonstick 18x13-inch baking sheet with butter or baking spray and parchment paper.
2. Combine dry ingredients. In a medium mixing bowl, whisk together the flour, both sifted cocoa powders, baking powder, and salt, until well-combined. Set aside.
3. Combine wet ingredients. In a large mixing bowl, using an electric hand mixer, beat together the butter, oil, white sugar and brown sugar until pale and fluffy, about 3 minutes. Add in the eggs and vanilla, then beat again until well-combined and smooth.
4. Add dry ingredients to wet. Add in the dry ingredients to the wet batter, then beat again until well-combined.
5. Bake. Transfer the batter into the prepared baking sheet. Smooth it out with a spatula. Place into the oven to bake for 11-12 minutes.
6. Slice. Remove from the oven, then immediately use a sharp knife to slice down the centre of the cookie. Allow to cool in the pan until fully cooled.
7. Invert. When fully cooled, place a parchment lined cooling rack (or the bottom of another baking sheet) on top of the cookie halves and carefully invert them.

Assemble the ice cream sandwiches:

1. Assemble. Remove the ice cream brick from the freezer and the pan, then carefully place it onto one of the cookie halves, then place the other cookie half on top.
2. Freeze. Lightly cover with plastic wrap, then transfer back into the freezer to harden for at least another 4 hours, or preferably overnight.
3. Slice. When hardened, use a sharp knife to slice into 12 bars. Enjoy immediately, or wrap each bar individually with plastic wrap, and place back into the freezer.