Easy German Apple Streusel Cake (Apfelstreuselkuchen)

Name: Easy German Apple Streusel Cake (Apfelstreuselkuchen)
Prep time: 15 min
Cook time: 45 min
Total time: 1 h
Cuisine: German
Ingredients: 150 g flour (5 oz (Germany type 405, UK plain flour, US pastry flour) ); 2 tsp baking powder; 50 g corn flour/ corn starch (1.7 oz); 1 tsp lemon peel; 1 pinch salt; 3 eggs ( (medium size, at room temperature)); 100 g butter (3.5 oz (at room temperature) ); 100 g granulated sugar (3.5 oz); 4 apples (about 1 kg/ 3.3 lbs - see post for which varities work best. ); 2 tsp fresh lemon juice; 100 g butter (3.5 oz at room temperature ); 150 g flour ( 5 oz); 2 tsp vanilla sugar ((alternatively 1 tsp vanilla extract) ); 1/2 tsp ground cinnamon