To make yeast dough, stir a pinch of sugar into warm milk and sprinkle with yeast.
Let stand 5 minutes or until frothy.
Stir gently to moisten any dry particles remaining on top.
Sift flour, remaining sugar and salt into a large bowl.
Melt butter; cool slightly.
Lightly beat butter, egg and lemon peel into yeast mixture.
Pour into flour mixture, combining to make a dough.
On a floured surface, knead dough lightly.
Cover and let rise in a warm place 1 hour.
Grease a 15-1/2x10-1/2-inch (39x27-cm) cake pan.
Knead risen dough lightly; roll out to fit cake pan.
Place dough in greased pan.
To make topping, place apples in overlapping rows on dough.
Let rise in a warm place 15 minutes.
Preheat oven to 400°F (205°C).
Sift flour, unflavored sugar and vanilla sugar into a large bowl.
Cut in butter with a pastry blender or 2 knives until mixture resembles crumbs.
Spread crumb topping over apples; sprinkle with currants.
Bake 25 to 35 minutes or until apples are soft and crumb topping is golden.
Cool slightly in pan then cut into slices.