Preheat the oven to 300°.

Line a 9-by-13-inch baking pan with parchment paper, leaving the long sides with longer pieces to be able to lift the brownies from the pan after cooking.

In a large saucepan, melt the butter with the unsweetened chocolate (OR oil/cocoa powder mixture) over low heat, stirring until melted.

In a large bowl, mix the sugar with the eggs, vanilla and salt. Add the melted chocolate-butter mixture and whisk until smooth. Add the flour and stir well. Fold in 1/2 cup each of the three chocolates.

Spread the batter evenly in the prepared pan.

Place the remaining 1/4 cup of each chocolate separately in 3 small microwave-safe bowls and melt. Using a spoon, drizzle the melted chocolates over the batter. Using a table knife, make swirls in the batter for a marbled effect.

Bake 35 minutes, rotating the pans halfway through, until the tops are shiny and the brownies are set. Do not over bake or the edges will be dry. These are a fudgy brownie.

Transfer the brownies to a wire rack to cool completely. Use parchment handles to lift from the pans and use a pizza cutter or large knife to cut into squares.