Preheat oven to 350 degrees F (175 degrees C). Butter 8 inch square baking pan.
Melt unsweetened chocolate, semisweet chocolate, and butter in a saucepan over low heat. Remove from heat.
Stir in brown sugar, white sugar, and honey; mix until sugar is dissolved. Beat in eggs, a little at a time. Add flour, salt, baking powder, vanilla, hazelnuts, cayenne pepper, and orange zest. Stir until blended well and mixture is glossy. Pour batter evenly into pan.
Bake until the surface looks dry and a toothpick inserted in the center has just a film of chocolate on it, 30 to 40 minutes. Cool on a rack about 12 minutes and cut into squares while still warm.