Custard:
Combine the milk, cornstarch, sugar, and eggs in a small saucepan over medium heat. Whisk continuously while the mixture heats up, until the eggs are completely incorporated.
Stir occasionally for 6 to 8 minutes, until the custard is thick enough to coat the back of a spoon; it should be around 160°F.
Remove from the heat and stir in the extract, then strain into a clean small pitcher. Cover with plastic and refrigerate to cool for at least an hour.
Dough:
Preheat oven to 350°F and combine the melted butter, milk, sugar, and yeast in a stand mixer bowl and stir until the yeast is dissolved. Add the salt and spices, then gradually mix in the flour until you have a dough that isn't too sticky to handle. Turn out onto a lightly floured surface and knead for several minutes, until the dough bounces back when poked. Cover with plastic and set somewhere warm to rise for about 1 hour or until doubled in size.
Once the dough has risen, punch it back down and divide into 10 equal pieces. Shape each of these into a slightly flattened ball and place on a baking sheet lined with parchment paper. Cover with plastic again and allow to rise for about 30 minutes more, until light and puffy.
Once the buns have risen, using a small glass or your thumb, press down and outward into the middle of each bun to form a hollow indent in the dough. Brush the buns all over with egg wash, THEN pour a little custard into each indent. Bake for 12 to 15 minutes, until the buns are a nice golden brown.
