In a heatproof bowl over a pan of gently simmering water combine chocolate, butter and sugar. Stir until all ingredients are almost melted. Remove the bowl from the heat and whisk vigorously with a spoon until the remaining small pieces of chocolate and butter are completely melted. Add eggs and vanilla and whisk for 2-3 minutes with a hand whisk. Add flour and stir gently until combined.
Pour half of the apple batter into the prepared bundt pan. Spread on top the chocolate batter. Cover with the remaining apple batter.
Bake for 50-60 minutes or until a wooden skewer inserted in the cake comes out clean.
Remove the pan from the oven and let cool for 30 minutes. Carefully run a butter knife around the edges of the cake. Invert the cake on top of a serving plate. Before serving sprinkle with powdered sugar.