On a lightly floured work surface, roll out the dough to a rough 8-by-12-inch rectangle. Cover with plastic wrap and let rest for 25 minutes. Lightly flour the rolling pin and roll out the dough to a 14-by-18-inch rectangle. Cover with plastic wrap and let rest again for 25 minutes.
Butter a 9-inch cast-iron skillet. Flour the work surface. Roll out the dough as thinly as possible, keeping a rectangular shape. Scatter 5 slices of the butter over two-thirds of the dough and sprinkle with 1/4 cup of the sugar. Fold the bare dough into the center and fold in the opposite side, like folding a letter. Dust with flour and roll out again as thinly as possible, retaining the rectangular shape. Top with another 5 slices of butter and 1/4 cup of sugar, folding, dusting with flour and rolling out the dough again as before. Repeat the process one last time, using the remaining 6 slices of butter and 1/4 cup of the sugar, folding and rolling it out as thinly as possible.
Fold in the corners of the dough to make a slightly round shape and transfer it to the prepared skillet. With a sharp knife, make six 2-inch-long cuts all the way through the dough in a starburst pattern. Cover the dough with plastic wrap and let rise for 45 minutes. Place a baking stone or baking sheet in the lower third of the oven and preheat to 450°.
Brush the dough with the melted butter and sprinkle with the remaining 2 tablespoons of sugar. Put the skillet on the stone in the oven and bake for 25 minutes, until richly browned on top. With a spatula, transfer the cake to a large, round plate and let rest for 30 minutes. Cut into wedges and serve.
The cake can be kept at room temperature overnight, but it’s best eaten the day it’s made.
This buttery cake and its caramel-like topping are a great showcase for a tangy-sweet tawny port or Madeira.