Step 1: Prepare the Dough

1. In a small saucepan, heat the milk over low heat until lukewarm. It should be warm to the touch but not hot (about 100–110°F or 37–43°C).
2. In a stand mixer fitted with the dough hook attachment, combine the flour, sugar, yeast, cardamom, and salt. Mix briefly to evenly distribute the ingredients.
3. Add the lukewarm milk and one of the eggs. Knead on low speed for 8 minutes until the dough starts coming together.
4. Add the butter, a few pieces at a time, and increase the mixer speed to medium. Knead for about 5 more minutes until the dough becomes smooth, elastic, and slightly sticky.
5. Transfer the dough to a lightly buttered bowl, cover with a tea towel, and let rise in a warm place for 1 hour or until doubled in size.

Step 2: Make the Filling

1. In a small bowl, mix the butter and brown sugar until well combined.
2. Roughly chop the frozen raspberries and set aside. Keeping them frozen helps prevent excess moisture when rolling the dough.

Step 3: Roll and Fill the Dough

1. Once the dough has risen, line two baking sheets with parchment paper.
2. Lightly flour a clean surface and use a rolling pin to roll out the dough into a 20 x 15-inch (50 x 38 cm) rectangle.
3. Spread the butter and brown sugar mixture evenly across the dough, making sure to reach the edges.
4. Sprinkle the chopped raspberries over the filling.
5. Starting from one long edge, roll the dough tightly into a 20-inch (50 cm) log.
6. Using a sharp knife, slice the log into 12 equal pieces and place them on the prepared baking sheets.
7. Cover the buns with a tea towel and let rise for 40 minutes.

Step 4: Make the Vanilla Custard

1. In a bowl, whisk together the egg yolks and sugar. Add the cornstarch and mix until smooth.
2. In a saucepan, heat the milk over low heat. If using a vanilla bean, scrape out the seeds and add them to the milk along with the pod. If using vanilla extract, add it later.
3. Heat the milk until just about to simmer, then remove the vanilla bean pod.
4. Slowly pour the warm milk into the egg mixture, whisking constantly to prevent curdling.
5. Pour the mixture back into the saucepan and cook over medium heat for 5–8 minutes, whisking continuously until the custard thickens.
6. Remove from heat and let cool slightly. If using vanilla extract, add it now.

Step 5: Fill and Bake

1. Preheat the oven to 400°F (200°C).
2. Using a small spoon, gently press into the center of each bun to create a small indentation.
3. Fill each indentation with 1 to 2 tablespoons of custard.
4. Whisk the remaining egg and brush it over the dough to give the buns a golden color when baked.
5. Bake for 15 minutes, or until golden brown.
6. Transfer to a wire rack to cool slightly. Dust with confectioners’ sugar before serving.