Preheat oven to 390°F (200°C) and grease and flour a 13x9-inch baking pan. Set aside.
In the bowl of a food processor fitted with the paddle attachment combine butter, brown sugar, and vanilla extract. Beat for 4 minutes at medium-high speed until the mixture is pale and fluffy. Scrape sides of the bowl with a rubber spatula as needed.
In a small bowl combine flour, baking powder, spices, and salt. Set aside.
Add the eggs, one at a time, to the sugar-butter mixture. Mix for 20 seconds after each addition.
Add orange zest and mix until combined.
With the mixer running on low add flour mixture and milk alternating. Mix for 1 minute until combined.
Transfer batter to the prepared baking pan and use a rubber spatula to smooth the top.
Bake for 25-35 minutes until a skewer inserted comes out clean with only a few crumbs attached.
Let cool completely in the pan on a wire rack.
Melt chocolate and spread over the cooled cake. Sprinkle with chocolate and let chocolate set before serving.
Store covered in a cool place. This cake keeps fresh for up to 5 days and tastes best on the 2nd day.