1. Combine flour and sugar 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). Line a 9.5-inch (24cm) round springform tin with baking parchment and set aside. Peel the mandarin oranges and remove any white residue from the skin of the mandarins.

3. Add the butter (or margarine), sugar, vanilla extract, and lemon zest to a large bowl and mix until light and fluffy. Add the eggs separately and mix well after each addition. Combine flour, baking powder, and salt – add together with the milk to the bowl and mix until just combined. Transfer about 1/3 of the batter to the springform tin and spread evenly – you only require a thin layer here. Place the mandarin oranges (about 2-3 slices still connected) evenly distributed on the batter, then cover them with the remaining batter. Smooth out the top and sprinkle the streusels on top. Bake the cake for about 40-45 minutes or until a wooden skewer inserted into the cake comes out clean. Take the cake out of the oven and let cool down completely on a wire rack.