1. Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius).
2. Place the margarine in a 9×13-inch (20×30-centimeter) baking pan and put the pan in the oven to allow the margarine to melt.
3. Meanwhile, melt the chocolate in a medium pot over very low heat so it doesn’t burn. Add sugar when almost melted, then stir until fully melted and remove from heat. The mixture should be warm to the touch, but not hot.
4. Add in eggs and vanilla, then flour, baking powder, and salt. Stir until fully combined.
5. When the margarine is fully melted, pour the batter into the margarine and spread gently to ensure the batter is even. Bake for 35–40 minutes, until firm to the touch.