In a small bowl, combine the plums, wine and honey; let stand 30 minutes.
Meanwhile, grease bottom and 1 in. up the sides of a 9-in. springform pan with 1 tablespoon butter. Sprinkle with 3 tablespoons hazelnuts. Place pan on a baking sheet; set aside.
Preheat oven to 375°. In a microwave, melt chocolate and remaining butter; stir until smooth. Cool to room temperature. In a large bowl, combine eggs, 2/3 cup sugar and milk. Add chocolate mixture. Combine flour, baking powder, salt and baking soda; gradually add to chocolate mixture just until combined. Transfer to prepared pan.
Drain plums, reserving 2 tablespoons liquid. Arrange plums over batter; drizzle with reserved liquid. Combine cinnamon and remaining sugar and hazelnuts; sprinkle over top.
Bake 35-40 minutes or until a toothpick inserted in the center comes out clean. Cool on a wire rack for 10 minutes. Carefully run a knife around the edge of pan to loosen. Serve warm or cold. Garnish with whipped cream and confectioners' sugar if desired.