In the bowl of a stand mixer, whisk together 1½ cups (188 grams) flour, granulated sugar, cornstarch, yeast, and salt by hand. Add hot milk; using the paddle attachment, beat at low speed until combined, about 1 minute. Add 3 eggs (150 grams) and egg yolks, and beat until combined, about 1 minute. With mixer on low speed, gradually add 1 cup (125 grams) flour, beating until a shaggy dough forms; scrape sides of bowl.
Switch to the dough hook attachment. Beat at medium-low speed until dough is smooth, elastic, and slightly tacky, 4 to 6 minutes; add up to remaining ¼ cup (31 grams) flour, 1 tablespoon (8 grams) at a time, if dough is too sticky.
With mixer on medium-low speed, add butter, 1 tablespoon (14 grams) at a time, to dough, beating until combined after each addition (6 to 7 minutes total). Beat at medium speed until a smooth, elastic dough forms, 10 to 12 minutes. Turn out onto a lightly floured surface; knead 5 to 8 times, and shape into a smooth round.
Lightly oil a large bowl. Place dough in bowl, turning to grease top. Cover and refrigerate overnight.
Lightly spray a 10-inch round fluted removable-bottom tart pan with baking spray with flour.
Turn out dough onto a lightly floured surface, and punch down dough. Roll dough into a 32-inch log, ensuring dough has the same thickness all over; taper ends of dough. Form log into a circle, overlapping and pinching together tapered ends, ensuring circle is evenly thick all around. (The hole in the center of dough circle should be about 6¼ inches.) Transfer dough to prepared pan. Cover and let rise in a warm, draft-free place (75°F/24°C) until doubled in sized, about 1½ hours.
Preheat oven to 375°F (190°C).
Lightly brush remaining 1 egg (50 grams) onto dough. Garnish with pearl sugars, if desired.
Bake until golden brown and an instant-read thermometer inserted near center registers 190°F (88°C), 12 to 15 minutes, rotating pan halfway through baking and covering with foil during final 5 minutes of baking to prevent excess browning. Let cool in pan for 10 minutes. Remove from pan, and let cool completely on a wire rack. Store in an airtight container for up to 3 days.