In a small bowl, whisk together the milk, egg, and instant dry yeast.
In a separate large mixing bowl, combine the flour, salt, sugar, and ground cardamom, if using. (The ground cardamom is optional for the cinnamon butter buns, but it gives them a subtle Scandi flavor.) Make a small well in the flour, pour in the milk mixture and stir together until it forms a rough dough.
Tip the dough onto a lightly floured surface and either knead by hand for 8 to 10 minutes or in a stand mixer fitted with a dough hook for 5 to 10 minutes, gradually working the cubes of butter into the dough as you knead until it is smooth.
Pop the dough back into the bowl, place a damp tea towel or shower cap over the rim and leave to prove until the dough has almost doubled in size, 1 to 1 1/2 hours. Alternatively, refrigerate the dough (still covered) for anywhere between 4 hours and overnight. It will still proof, just more slowly, and the dough will be much easier to handle.
Tip your puffy dough onto a lightly floured work surface and roll it out into a 12- by 8-inch (30- by 20-cm) rectangle that is roughly 1/4 inch (5 mm) thick. Transfer the dough to a rimmed baking sheet and put it in the fridge for about 10 minutes. (Chilling firms up the dough, which makes it easier to spread over the butter filling during the next stage.) Meanwhile, prepare your chosen butter filling.