This recipe -- a tradtional cake from the Brittany region of France -- is brought to us by renowned pastry chef Florian Bellanger, of New York City's Fauchon.
In a small bowl, combine mineral water and salt. Let stand until salt has dissolved. Meanwhile, melt 2 tablespoons butter in a small saucepan over medium-low heat.
In the bowl of an electric mixer fitted with the dough hook, combine flour and the melted butter on low speed. Add water-and-salt mixture, and continue to mix until well combined, about 2 minutes. Add yeast, and mix for 1 minute more.
Cover the bowl with plastic wrap, and let rise in a warm place until doubled in size, about 1 hour. Punch the dough down, wrap in plastic, and place on a baking sheet. Chill for 2 hours in the refrigerator.
Meanwhile, on a Silpat (a French nonstick baking mat) or parchment paper, roll the remaining 1 pound butter into a 1/2-inch-thick rectangle. Wrap in parchment paper, and return to refrigerator until chilled, about 30 minutes. Remove the chilled dough from the refrigerator. On a lightly floured surface, roll out the dough to an 18-inch square. Center the chilled butter rectangle on the dough so that each side of the butter faces a corner of the dough. Fold the corners of the dough over the butter to enclose. Seal the edges by pinching them together. Roll the dough into a 24-by-8-inch rectangle. Fold the dough into thirds, aligning the edges carefully and brushing off any excess flour. (The object is to ensure that the butter is distributed evenly throughout so that the pastry will puff evenly when baked.) Wrap the dough in plastic, and chill for 20 minutes; this completes one turn.
Repeat process once, then repeat process twice, dusting the work surface and the dough with sugar, and using 1 1/2 cups for each turn. You will now have completed four turns.
Using a pastry brush, brush 15 ring molds (3 1/2 by 3/4 inches) with melted butter. Transfer to prepared baking sheets, and set aside. Remove dough from refrigerator. On a lightly sugared surface, roll the dough into a 1/2-inch-thick rectangle. Cut into 15 squares (4 1/4 inches). Fold up the corners of one square toward the center; repeat process. Lightly press to adhere. Turn square over, and gently coat with sugar. Invert, and place in a prepared ring mold. Repeat with remaining squares.
Preheat oven to 425 degrees. Let rise in a warm place until puffed, 30 to 40 minutes. Bake until golden brown, 35 to 40 minutes. Immediately remove ring molds, and place on a wire rack until completely cooled.