1. Combine flour, sugar, and salt for the streusels in a bowl. Add the butter in small pieces and rub everything between your fingers to create different-sized streusels. Place in the fridge until needed.

2. Preheat the oven to 180°C (350°F). Extend an adjustable baking frame* to a size of approx. 26x20cm, secure it, and set it on a baking sheet lined with baking parchment. Set aside.

3. Add the butter and sugar for the cake batter to a large bowl and mix until light and fluffy. Add the eggs one by one and mix well after each addition. Next, add the Skyr and vanilla extract and mix in. Combine flour, baking powder, and salt – add to the bowl and mix until just combined.

4. Peel and core the apples, then cut them into thin slices. Drizzle the apples with lime juice and sprinkle them with the brown sugar and ground cinnamon – mix to combine. Add the apples to the bowl with the cake batter and fold them in. Transfer the batter to the prepared baking frame and spread it evenly inside the frame. Distribute the streusels evenly on top and sprinkle everything with coarse sugar. Bake the cake in the preheated oven for about 45-50 minutes – or until a wooden skewer inserted into the cake comes out clean. Take the cake out of the oven and let it cool down completely inside the baking frame. Cut the cake into bars and serve with some whipped cream or vanilla custard.