Grease a 9-inch round cake pan with butter, then dust it with cocoa powder. This will not only prevent the cake from sticking to the pan but also add an extra layer of chocolate flavor for a truly indulgent experience. Preheat your oven to 350°F/175°C.
In a large bowl, utilize an electric mixer to cream together the butter and sugar until they reach a light and fluffy consistency. Once achieved, introduce the salt and baking powder into the mix and stir until fully combined.
Take your time adding each egg one by one, ensuring that each egg is fully incorporated into the batter before adding the next. This technique results in a smoother and more consistent texture for the cake. Gradually mix in the cocoa powder until it becomes one with the butter mixture.
In a small saucepan, melt your choice of chocolate over low heat, stirring constantly until it reaches a smooth and velvety consistency with no lumps. Allow the chocolate to cool slightly before adding it to the batter and mixing until fully incorporated.
Slowly mix in the cornstarch, taking care to incorporate it evenly throughout the batter. Once fully integrated, pour the batter into your prepared pan and use a spatula to smooth out the top for an even layer.
Bake the cake for 20-25 minutes, or until a toothpick inserted in the center comes out clean. Once done, remove it from the oven and let it cool completely before removing it from the pan.
To make the ganache, place finely chopped chocolate in a heatproof bowl. In a small saucepan, heat heavy cream until it reaches a gentle simmer. Pour the hot cream over the chocolate and let it sit for 1 minute before stirring until it becomes smooth and glossy.
Spread the ganache over the cooled cake, using a spatula to create an even layer. Allow the ganache to cool and set for 15-20 minutes before dusting the top with cocoa powder. Slice, serve, and indulge in the rich and decadent flavors of Lisbon chocolate cake.