Combine flour, cornstarch, salt, and butter in a large bowl. With your fingertips, mix until a coarse meal forms. Add the water, all at once, and knead by hand until a ball forms.
Transfer to a bowl of an electric mixer with a dough hook and knead until smooth and moist, about 5 minutes, or knead by hand about 15 minutes. Press dough into a 6 by 4 inch rectangle. Wrap with plastic and chill 4 hours or overnight.
Soften butter block with your hands or with an electric mixer until pliable enough to roll like doughnut not too soft. The key to rolling butter and dough is they must be warm enough to roll, but not warm enough to combine. The two parts should always remain separate.
On a floured board, roll cold dough, from center out, to form a 9 by 6 inch rectangle, keeping the thickness uniform, about 1/2inch thick. Align dough so the length is parallel to the counter.
Imagine the dough divided in thirds along its width and spread butter over middle and right thirds, leaving a 1/2 inch margin uncovered on all sides. Fold dough in thirds, first left over middle, then right over left. Press edges together to seal in butter and press out any air.
Turn folded dough clockwise a quarter turn. You have now completed one turn. To make second turn, roll dough out again to a 9 by 6 inch rectangle. Fold in thirds again. As a reminder of turns completed, press 2 dots in dough with your fingertips. Wrap in plastic and chill 1/2 hour.
Roll, fold, and turn dough two more times. Press the dough with four dots, wrap, and chill 1/2 hour. Repeat the same procedure 2 more times, for a total of 6 turns. Wrap well in plastic and store in refrigerator overnight. Puff pastry may be frozen as long as 3 weeks.
To make almond cream, process sugar and almonds in a food processor until fine. Add butter, 1 tablespoon at a time, processing after each addition until smooth. Add remaining ingredients and process until smooth. Cover and refrigerate until ready to assemble. The almond cream may be made up to 4 days in advance.
1 egg, lightly beaten
Divide puff pastry in half. On a lightly floured board, roll each piece to a 10 inch square. Set aside in refrigerator 1/2 hour.
After chilling, assemble tart on a sheet pan lined with parchment paper. Lightly roll each piece so it returns to a 10inch square. Place one pastry square on pan. Mound softened almond cream in a 5 inch circle about 2 inches high in center of square. Brush egg on exposed pastry.
Place second pastry square over almond cream, so corners align. Pressing out from center to release any air, seal in cream by pressing together top and bottom pastry. Cut a small hole in center of top layer using a paring knife. Press out excess air. Chill an additional hour or, preferably, overnight.
Preheat oven to 425 degrees F.
Place in a 10 inch round bowl over tart and, using a sharp paring knife, trim excess pastry to form a 10 inch circle. Do not drag knife. Press edges again to seal and carefully brush top with egg. Using a paring knife, make a pinwheel pattern by scoring dough from center out.
Bake, on lined sheet pan, 20 minutes. Reduce heat to 350 degrees F and bake an additional 1/2 hour. Serve immediately.
c.1997, M.S. Milliken & S. Feniger, all rights reserved