Place 3 cups of flour, milk, sugar, salt, egg, and yeast in a standing mixer and mix well. Knead the dough with the hook attachment for about 10 minutes. If the dough feels too sticky then add more flour, 1 tablespoon at a time. Turn the dough out onto a clean lightly floured surface. Add the grated butter little by little as you knead it into the dough. Make sure all the butter is incorporated and you have a smooth and elastic dough. Place in a large oiled bowl covered with a towel and place in a warm spot in the kitchen. Let the dough rise until it’s double its original size—about 1 hour. Take it out of the mixing bowl and use a rolling pin to roll it into a 12”x16” rectangle. Spread the dough with the filling ingredients: butter, sugar, cinnamon, and marzipan. Roll dough into a log lengthwise and cut into 12 equal pieces. Place on a baking tray about 1⁄2” apart and cover with parchment paper. Let the buns rise for 1 hour. Preheat the oven to 350°F. Brush with egg wash, and bake for about 20 minutes or until golden brown. Whisk together maple syrup and rum, and brush the buns with the glaze as soon as they get out of the oven. Cool on a wire rack.