Heat oven to 170°C. Grease bottoms and sides of two 23cm round cake pans. Line bottoms of pans with parchment paper.
Make cake mix as directed on box, using water, oil and eggs. Carefully fold in 170 grams chocolate chips into batter and divide evenly between two cake pans. Bake as directed on box for 23cm rounds. Cool 10 minutes. Run knife around sides of pans to loosen cakes; remove from pans to cooling racks. Cool completely, about 30 minutes.
In 2-liter saucepan, heat chocolate chips and butter over low heat, stirring constantly, until chocolate is melted; remove from heat. Stir in whipping cream. Refrigerate 30 to 40 minutes, stirring frequently, just until thick enough to mound and hold its shape when dropped from a spoon. (If filling becomes too thick, microwave on High 10 to 15 seconds to soften.)
Place 1 cake layer on serving plate; spread with 150 grams filling. Top with remaining cake layer. Reserve 2 tablespoons filling. Frost side and top of cake with remaining filling. Drizzle with 2 tablespoons reserved filling. (If filling is too thick to drizzle, microwave on High 5 to 10 seconds to soften.) Garnish with hazelnuts.