Preheat the oven to 350°F (180°C) and grease a medium-sized baking pan.
In a bowl, combine the rolled oats, flour, baking powder, and grated lemon zest.
In another bowl, cream together the butter and sugar until light and fluffy. Then, add the eggs one at a time, mixing well after each addition.
Gradually add the dry ingredients to the butter mixture, mixing until just combined.
In a separate bowl, combine the cottage cheese, sour cream, vanilla sugar, and one egg. Mix until smooth.
Press half of the oat mixture into the bottom of the prepared pan. Spread the cottage cheese filling evenly over the crust.
Crumble the remaining oat mixture over the filling, and gently smooth it out.
Bake the pie for 35-40 minutes, or until golden brown on top.
Let the pie cool completely before dusting with powdered sugar and slicing to serve.
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