In a microwave-safe bowl, warm the milk in the microwave for about 45 to 60 seconds, until it's between 98°F to 105°F.
Mix the warm with the granulated sugar and active dry yeast. Set aside for 10 minutes until it becomes frothy on top.
Whisk the yeast mixture with the eggs.
Sprinkle the flour, cinnamon, optional cardamom, and salt on top of the wet ingredients, and mix with a large spoon until it starts to form a sticky dough.
Add the softened butter and continue mixing until there's no dry flour visible.
Lightly dust a clean work surface with flour, and knead the dough by hand for 8 minutes, until the dough is smooth and elastic. Add a little more flour as you knead the dough to prevent it from sticking, but try to only add as little as possible.
Shape the dough into a ball. Place the dough ball in a clean mixing bowl, and cover the top of the bowl with a large plate or kitchen towel. Let it rest and rise in a warm place for about 2 hours, until it doubles in size.
Grease a Bundt pan. Set aside.
Mix the dry filling ingredients in a small bowl (½ cup brown sugar, ⅓ cup granulated sugar, 1 tablespoon cinnamon, and ½ teaspoon nutmeg). Set aside.
Melt the 6 tablespoons of butter for the filling in a medium bowl in the microwave. Set aside.
Tear off a small piece of dough (about 35 grams or 1½ inches in diameter) and roll it in your hands to form a ball. Dip the dough ball in melted butter, then in the cinnamon sugar. Place it in the greased Bundt pan. Continue shaping, coating, and arranging the dough in the Bundt pan. (I end up with around 35 balls in total.) Cover the Bundt pan with a clean kitchen towel and let it proof at room temperature for 30 minutes.
Preheat the oven to 350°F. Position your oven rack in the lower third (so the buns in at the top of the Bundt pan don’t burn.)
Bake for 30 to 35 minutes, until golden brown on top and a kitchen thermometer reads at least 190°F when inserted into the middle of the bread. Cool for 10 minutes in the pan, then flip the Bundt pan upside down on a serving plate. Enjoy!