In a large bowl, whisk together the flour, sugar, and salt.

In a separate bowl, combine the warm water and yeast. Let it sit for about 5 minutes until the yeast is foamy.

Add the warmed milk and melted butter to the yeast mixture.

Pour the wet ingredients into the dry ingredients and stir until a shaggy dough forms.

Turn the dough out onto a lightly floured surface and knead for 5-7 minutes until it becomes smooth and elastic.

Place the dough in a lightly oiled bowl, cover it with plastic wrap, and let it rise in a warm place for 1-1.5 hours, or until doubled in size.

While the dough is rising, prepare the butter block. Place the cold butter between two sheets of parchment paper.

Using a rolling pin, pound and roll the butter into a 6-inch square. Keep it cold!

Once the dough has doubled, punch it down to release the air.

On a lightly floured surface, roll the dough into a 12-inch square.

Place the cold butter block in the center of the dough at a diagonal.

Fold the corners of the dough over the butter like an envelope, completely encasing the butter.

Gently roll the dough into a 20x10-inch rectangle. Be careful not to press too hard, or the butter might break through.

Fold the dough in thirds, like a letter. This is your first 'turn.'

Rotate the dough 90 degrees, and roll it out again into a 20x10-inch rectangle.

Fold it in thirds again. This is your second 'turn.'

Wrap the dough tightly in plastic wrap and chill in the refrigerator for at least 30 minutes.

Repeat steps. Gently roll the dough into a 20x10-inch rectangle. Fold the dough in thirds. Rotate the dough 90 degrees, and roll it out again into a 20x10-inch rectangle. Fold it in thirds again.

Wrap the dough again and chill.

In a small bowl, combine the brown sugar, cinnamon, and softened butter. Mix until it forms a paste.

On a lightly floured surface, roll the chilled dough into a large rectangle, about 1/4 inch thick.

Spread the cinnamon mixture evenly over the dough, leaving a small border along one edge.

Starting from the opposite edge, tightly roll the dough into a log.

Use a sharp knife to slice the log into 1-1.5 inch thick rounds.

Place the cinnamon swirl rounds on a baking sheet lined with parchment paper.

Cover them loosely with plastic wrap and let them proof (rise) for another 30-45 minutes, or until they are puffy.

Preheat your oven to 375°F (190°C).

Bake the croissants for 18-20 minutes, or until they are golden brown and cooked through.

Let them cool slightly on a wire rack before serving.