Preheat the oven to 350°F / 180°C and prepare a 8 inch / 20 cm square brownie pan by lining it with parchment paper.
Melt the chocolate and butter in a double boiler or microwave until smooth.
In another large bowl, place the powdered sugar, brown sugar, eggs and vanilla extract and beat with an electric whisk for 1-2 minutes until smooth and fluffy.
Add the liquid chocolate and butter mixture and mix again with an electric whisk for 1 minute.
Add the dry ingredients, flour, cocoa powder and salt and mix with a spatula or wooden spoon until smooth.
Pour the mixture into the brownie pan and bake for about 15-20 minutes. Insert a knife blade into the center of the brownie, it should come out with a few traces of batter. The brownie is ready when the center is still soft and will firm up as it cools.
Let the brownie cool completely in the pan.
Chop the dark chocolate and place it in a container.
Heat the heavy cream in a saucepan and then pour it into the chocolate bowl.
Mix with a spatula until you have a smooth ganache.
Pour the chocolate ganache over the brownie and spread it out to form a smooth layer.
Place smarties on top and place the brownie in the fridge for at least 1 hour before unmolding.
Cut the brownies into 16 equal pieces with a large knife.