Preheat the oven to 375 degrees.
Cut a piece of parchment paper to fit the bottom of a 8-inch cake pan. Spray the entire pan including the parchment paper with nonstick cooking spray.
To make the cake, put the semisweet chocolate chips and butter in a microwave-safe bowl and heat for about one minute or until the butter and chocolate chips are melted. Stir until the mixture is smooth. If you prefer to use the stovetop, heat the semisweet chocolate chips and butter in a pan on low heat. Stir until melted and combined.
Add the sugar, salt and the extract or the zest and stir to combine.
Add the eggs and stir to combine. Fold in the cocoa powder and stir until combined but be careful not to overmix.
Pour the cake batter into the cake pan and bake for 23-25 minutes or until the center is firm but fudgy.
Cool the cake on a wire rack for at least 30 minutes or until it reaches room temperature.
Use a knife to loosen the edges. Carefully flip the cake onto the serving platter.
To prepare the ganache, place the milk chocolate chips and the extract (if using) in a medium bowl. Warm the heavy cream in a medium pan on the stovetop or for 30 seconds in a microwave-safe bowl in the microwave. Pour the cream over the chocolate and stir until the chocolate is melted.
Pour the warm ganache over the cake. If you're using orange zest, sprinkle the zest over the ganache before it cools.