For the crumb topping: In a small bowl, combine 1 tablespoon flour, 1/2 cup brown sugar and 2 tablespoons butter. Stir in 1/2 cup nuts and 2 cups chocolate chips. Set aside.
Preheat oven to 180 degrees C. Grease and flour a 20x30cm pan. In a small bowl, combine 2 cups flour, bicarb soda, baking powder and salt. Set aside.
In a large bowl, cream 125g butter, 1 cup sugar and 1 teaspoon vanilla until light and creamy. Add eggs one at a time, beating well after each addition. Add flour mixture alternately with sour cream. Beat until smooth. Fold in 1 1/2 cups of chocolate chips.
Spread batter into pan. Sprinkle topping evenly over batter. Bake for 45 to 50 minutes, or until a toothpick inserted into cake comes out clean. Cool completely.