In a large mixing bowl, combine all-purpose flour, granulated sugar, and active dry yeast. Create a well in the center and add whole milk and one egg. Mix until a shaggy dough forms, then knead on a floured surface for about 5-7 minutes until smooth and elastic.

Shape the kneaded dough into a ball and place it in a lightly oiled bowl. Cover and let it rise in a warm area for about 1 hour, or until doubled in size.

Once the dough has risen, punch it down gently. Roll it out into a rectangle approximately ¼ inch thick. Spread softened unsalted butter evenly over 2/3 of the surface, leaving one edge bare. Fold the unbuttered edge over the buttered portion.

Roll the dough out again to ½ inch thickness. Cut it into squares and place about one tablespoon of your favorite filling in the center of each square. Fold corners towards the center and pinch to seal.

Arrange the filled dough in greased muffin tins, cover with a cloth, and allow them to rise for another 30 minutes.

Preheat your oven to 375°F (190°C). Bake the cruffins for about 25 minutes, or until golden brown.