Melt the chocolate, oil, and butter/margarine using a double boiler method (heat a small pot with enough water to boil, turn off the heat, place a heatproof bowl or larger stainless steel container over the pot containing the chocolate, oil, and butter) until melted and smooth. Set aside to cool to room temperature.

Preheat the oven to 320°F (160°C) with top and bottom heat. Whisk the powdered sugar and eggs until the sugar is completely dissolved in the eggs. Then add the melted chocolate mixture that has cooled to room temperature and mix until well combined.

Add the sifted flour and cocoa powder to the batter. Mix well with a spatula until there are no lumps.

Pour into the baking pan, insert the dividers, and add toppings as desired. Bake in the oven for 30-45 minutes (adjust according to your oven). Once done, remove the dividers and lift the brownies out by pulling the parchment paper over the sides of the pan, then cool on a cooling rack.

Tips:
* For moister/fudgier brownies, bake for about 25-30 minutes. Perform a toothpick test; if it comes out with a few moist crumbs but not wet, and the top is firm when pressed, they are ready to be removed.
* The size of the baking pan also affects baking time. A smaller pan means thicker batter and longer baking time, while a larger pan means thinner batter and shorter baking time.