1. Start with the blueberry sauce by adding the blueberries and lemon juice to a small saucepan and heat it up. Let the blueberries simmer (over medium-high heat) for about 10 minutes while stirring often. The blueberries should have broken down, and the juices should have thickened nicely. Remove from the heat and let cool down a bit, then press through a fine-mesh sieve to get rid of the blueberry skins. Let cool down completely.

2. For the buttercream, add a small amount of the milk (3-4 tablespoons) to a bowl and mix with cornstarch, sugar, salt, and vanilla extract until well combined. Add the rest of the milk to a saucepan and bring to a boil. Add the cornstarch mixture and stir in while stirring constantly until you get a smooth and thick custard. Let bubble for a moment, then remove from the heat and place a piece of plastic wrap directly on the custard to prevent skin from forming. Let cool down completely. You can do the custard and blueberry sauce a day in advance.

3. Preheat the oven to 350°F (180°C). Line a muffin tin with paper liners and set aside.

4. For the batter, add both sugars, the egg, sunflower oil, apple juice, and vanilla extract to a large bowl and mix until well combined. Mix flour, baking powder, baking soda, cinnamon, cardamom, and salt in a separate bowl. Add these dry ingredients to the bowl with the wet ingredients and mix until just combined – do not overmix the batter! Fill the paper liners about 2/3 with the batter. Bake the cupcakes for 18-20 minutes or until a toothpick inserted into the centers of the cupcakes comes out clean. Take the cupcakes out of the oven and let cool down on a wire rack.

5. While the cupcakes are cooling down, you can finish the buttercream. Add the butter and confectioners’ sugar to the bowl of a stand mixer and mix until very light and fluffy – about 4-5 minutes. Gradually add the custard (make sure it has the same temperature as the butter) and mix in. When all custard is incorporated, continue mixing for 2-3 minutes to get a very soft and smooth buttercream. Add the blueberry sauce (keep some for decorations) and mix until well combined – this might take another 2-3 minutes. If the buttercream seems too thick, you can add a little heavy cream and mix in.

6. Fill the buttercream into a piping bag with a large star tip and pipe a good amount of buttercream on each cooled cupcake. Decorate with some leftover blueberry sauce and fresh blueberries.