Preheat oven to 350°F / 175°C. Grease a springform pan (Ø 23cm) and line the bottom with parchment paper.
Prepare the crumble. Prepare the topping by stirring together the oats, brown sugar and cinnamon. Add the butter and use your fingertips to incorporate, until it form crumbles. Set aside.
Prepare the cake. In a medium bowl, whisk together the flour, baking powder, salt and cinnamon. Set aside.
In a large bowl, with an electric mixer or the bowl of a stand mixer with the paddle attachment, beat together the eggs and sugar, until light and creamy, about 3 minutes.
With mixer on low, slowly add the flour mixture and mix until JUST combined. Do not over mix!
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