Preheat the oven to 375°. Butter and flour a shallow 8-inch cake pan.
In a medium bowl, using an electric mixer, beat the almond paste with the sugar until blended. Beat in the butter and then the egg. Sift the cocoa powder with the cake flour. Add the dry ingredients to the batter alternately with the milk.
Scrape the batter into the prepared pan and bake for 25 minutes, or until a toothpick inserted in the center comes out clean. Let cool in the pan for 10 minutes, then unmold onto a rack to cool completely.