Hungarian Coffee Cake

Make the streusel

1. In a small bowl, mix the flour, superfine sugar, brown sugar, cinnamon, salt, and butter with your fingers until combined and crumbly.

Make the coffee cake

1. In another small bowl, mix the brown sugar, superfine sugar, and cinnamon.
2. Lightly dust a work surface with flour. Place the dough on the work surface and dust lightly with flour. Roll out the dough into an 18-by-10-inch rectangle, with a long side facing you. Brush the well-softened butter over the surface, leaving a 1-inch border on all sides. Sprinkle the sugar mixture over the butter.
3. Starting at the top of the rectangle, tightly roll up the dough, leaving an empty border along the edge. Brush the empty border of dough with the beaten egg and pinch the seam closed. Gently roll the dough back and forth underneath your palms to seal the seam securely.
4. Push the ends in on each side and roll again to stretch the log to 18 inches. Fold the dough roughly in half to form a U-shaped curve, with one side 3 inches longer than the other.
5. Using the side of your hand, press a dent into the dough at its bend. Fold the longer length of dough over and around the shorter length twice to make 2 humps.
6. Twist the dough lengths to create a third hump and tuck the two ends under the loaf. You should have a loaf about 9 inches long with 3 humps.
7. Generously butter a 9-by-5-by-3-inch loaf pan. Transfer the dough to the pan, making sure that the ends are secured under it. Gently brush some butter all over the top of the dough. Sprinkle the streusel over the top, patting it gently so it adheres. (Don’t worry if some of the streusel falls into the corners or sides of the pan.) Place the loaf pan on a half-sheet pan or rimmed baking sheet.
8. Choose a warm spot in the kitchen for proofing. Slip the pan with the loaf pan into a tall kitchen plastic bag. Place a tall glass of hot water on either side of the loaf pan to keep the bag from touching the dough. Wave the bag to inflate it and tightly close it, trapping air in the bag. Let stand until the loaf rises 2 inches above the rim of the pan, about 45 minutes.
9. Meanwhile, position an oven rack in the center of the oven and preheat it to 350°F (177°C).
10. Carefully remove the glasses from the bag, then remove the pan. Bake the coffee cake on the half-sheet pan until the top is deep golden brown, the dough in the crevices looks fully baked, and an instant-read thermometer inserted into the center of the cake reads at least 195°F (91°C), 45 to 60 minutes. (It's sorta tricky to tell when the loaf is done just by visual cues alone, so we encourage the use of a thermometer.) If the loaf threatens to darken too much, cover the top loosely with foil.
11. Cool in the pan on a wire rack for 10 minutes. Carefully unmold the cake onto the rack and let cool completely, right side up.
12. To serve, reach for a serrated knife to cut the cake into slices.