1. Add the cornstarch, sugar, and vanilla extract for the vanilla custard to a small bowl and mix with 2-3 tablespoons of the milk. Add the remaining milk to a pot, mix in the egg, and then heat slowly until the milk starts to boil. Next, add the cornstarch mixture while constantly mixing until the pudding starts to thicken. Remove from the heat, let cool down for about 10 minutes, then add the sour cream and mix in. Place a piece of plastic wrap directly on top of the custard to prevent skin from developing, then let it cool completely.

2. Add the flour for the dough, the sugar, and the salt to a large bowl and mix to combine. Make a well in the center, pour in the lukewarm milk, crumble the yeast into small pieces and add to the milk. Mix a bit and then let it sit for about 5 minutes. Add the butter, lemon zest, and egg and knead for about 6-7 minutes until you get a nice smooth and slightly sticky dough. Cover the bowl and let the dough rise in a warm place for about 45-60 minutes – it should almost double in size.

3. Knock the air out of the dough and divide it into two equal pieces. Wrap one piece in plastic wrap and place it in the fridge until needed. Divide the other dough piece into six equal portions and shape them into balls. On a floured surface, roll out each ball thinly into an oval with about 20x12cm in length/width. Using a sharp knife, cut on one side of the dough oval several times into the dough and spread about 1-2 tablespoons of the custard filling on the opposite side. Fold the dough over the filling and roll up the croissant – the side with the cuts should be facing upwards. Bend the tips slightly to the front to form the croissant. Place the croissants on a baking sheet lined with baking parchment with enough space in between. Cover with a clean kitchen towel and allow to rise for about 15-20 minutes.

4. While the croissants are rising a second time, preheat the oven to 180°C (350°F). Brush the risen croissants gently with milk, sprinkle with some sliced almonds, and then bake for about 25 minutes. The croissants should have puffed up nicely with a nice golden color. Remove from the oven and let cool down on a wire rack. While the first batch is in the oven, you can prepare the remaining croissants with the remaining dough from the fridge. Dust the vanilla croissants with some confectioners‘ sugar before serving.