Melt the butter, oil and cocoa

• Melt the butter in a saucepan over the stovetop.
• Add the oil and cocoa powder and let it bubble briefly.
• Turn off the heat and remove the pan from the stovetop.

Add the dark chocolate

• Stir the chopped dark chocolate into the warm mixture.
• Let it cool slightly.

Whip the eggs and sugar

• Whisk the eggs and sugar vigorously by hand until pale pastel yellow.

Combine the wet ingredients

• Add the vanilla and the melted chocolate mixture to the whipped eggs.
• Whisk until smooth.

Fold in the dry ingredients

• Add the tapioca flour and salt and fold in.
• Fold in the chocolate chips.

Bake the brownies

• Grease an 8x8 dish, line it and grease again.
• Pour in the batter and bake at 180 °C.