Marble Milk Bread

In a small bowl, whisk together warm milk and yeast. Let stand until foamy, 5 to 10 minutes. In the bowl of a stand mixer fitted with the paddle attachment, place 1 cup (125 grams) flour, sugar, and 1¼ teaspoons (3.75 grams) salt. With mixer on low speed, add Tangzhong and yeast mixture, beating until combined, about 1 minute. Beat in 1 egg (50 grams) until combined. Add 1 cup (125 grams) flour and butter; beat just until combined, about 1 minute.

Switch to the dough hook attachment. Beat at low speed until a soft, elastic, just barely sticky dough forms, 8 to 12 minutes; add up to remaining ¼ cup (31 grams) flour, 1 tablespoon (8 grams) at a time, if dough is too sticky. (Dough should pass the windowpane test but may still stick slightly to sides and bottom of bowl.) Turn dough out onto a clean surface, and shape into a ball.

Lightly oil a medium bowl. Place dough in bowl, turning to grease top. Cover and let rise in a warm, draft-free place (75°F/24°C) until doubled in size, 40 minutes to 1 hour.

Punch down dough; cover and let stand for 10 minutes. On a lightly floured surface, roll dough into a 14×7-inch rectangle, one long side closest to you. Place Chocolate Sheet in center of rectangle, making sure there is an even amount of dough on either side of Chocolate Sheet. Fold ends of dough over center of Chocolate Sheet, gently pulling and pinching top and side seams to completely seal. Lightly sprinkle work surface with flour. Roll dough into 16×7-inch rectangle, lifting dough from work surface every few rolls to make sure it is not sticking and reflouring work surface as needed. Fold dough into thirds, like a letter. Gently roll to join folds. Rotate dough 90 degrees, and repeat procedure once (creating a total of 2 folds).

Preheat oven to 375°F (190°C). Spray an 8½×4½-inch loaf pan with baking spray with flour. Line pan with parchment paper, letting excess extend over sides of pan.

On a lightly floured surface, roll dough into a 10×9-inch rectangle, one short side closest to you. Using a sharp knife or pizza cutter, cut 6 long strips (1⅓ inches wide) along one short side, leaving ½ inch at top intact.

Separate strips into 2 groups of 3. Label first 3 strands A, B, and C. Label second group of strands D, E, and F. Fold C over E. Fold D over C and B. Fold B under E. Fold F over C and under B. Weave A over D, under E, and under F. Weave C over B and under A. Weave D over E, under F, and under C. Weave B over A and under D. Weave E over F, under C, and under B. Weave A over D, and tuck under. Weave F over C, and tuck under. Pinch together ends, and tuck under. Tuck sides of dough under, and place on prepared pan. Cover and let rise in a warm, draft-free place (75°F/24°C) until doubled in size and dough holds an indentation when poked, 40 minutes to 1 hour.

In a small bowl, whisk together remaining 1 egg (50 grams) and remaining ¼ teaspoon salt. Lightly brush top of loaf with egg wash.

Bake until deep golden brown and an instant-read thermometer inserted in center registers 190°F (88°C) to 200°F (93°C), 40 to 50 minutes, loosely covering with foil after 20 to 25 minutes of baking to prevent excess browning. Let cool in pan for 15 minutes. Remove from pan, and let cool completely on a wire rack.

In a small saucepan, whisk together milk and flour. Cook over medium-low heat, whisking constantly, until thickened, whisk leaves lines on bottom of pan, and an instant-read thermometer registers 149°F (65°C). Transfer to a small bowl, and let cool completely before using.

Fold a sheet of parchment paper in half. On outside of one half, using a permanent marker, draw a 7-inch square.

In a small microwave-safe bowl, heat chocolate and butter on high in 10-second intervals, stirring between each, until melted and smooth (about 45 seconds total).

In a medium saucepan, whisk together flour, cocoa, and cornstarch. Add sugar, milk, and egg white, whisking until smooth. Stir in chocolate mixture. Cook over medium to medium-low heat, whisking constantly, until mixture forms a very thick, dense, slightly less shiny paste that can be formed into a ball in pan, 10 to 20 minutes. (See Note.) Remove from heat, and let cool for 3 to 4 minutes.

Open parchment, and spoon paste over template; press and spread to form a 7-inch square. Fold parchment to cover paste, and freeze until ready to use.