Preheat oven to 175C (350F).
Grease an 8 x 12-inch baking pan or line with baking paper.
Combine melted butter, oil and sugar together in a bowl. Whisk well to combine. Add the eggs and vanilla and beat until lighter in colour.
Sift in flour, cocoa powder, and salt. Fold the dry ingredients into the wet ingredients until just combined.
Fold in ¾ of the chocolate chips. Pour batter into prepared tin, smoothing out the top evenly and topping with the remaining chocolate chips
Bake for 20-25 minutes