1. Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius). Grease an eight-inch (20-centimeter) square pan.
2. Melt chocolate and butter over low heat in a small saucepan. Stir occasionally. When the mixture is smooth, remove from heat and set aside.
3. Beat eggs and vanilla in a medium bowl. Add the melted chocolate mixture and whisk to combine (mixture will thicken). Add the remaining ingredients and stir with a spatula or a wooden spoon.
4. Pour the batter into the prepared baking pan. Smooth the top and bake for about 20-25 minutes, until set.