Make the strawberry filling: Add strawberries, sugar, lemon juice, and salt to a small saucepan over medium heat. Cook, stirring, until the berries release juices and soften, about 5–7 minutes. Stir in the cornstarch slurry and simmer 1–2 minutes more until thick and glossy. Transfer to a bowl, cool to room temperature, then chill.

Mix the dough: In a large bowl, whisk flour, sugar, yeast, and salt. In a separate bowl, whisk milk, egg, and vanilla. Pour wet into dry and stir until a shaggy dough forms. Knead by hand 2–3 minutes just to bring it together. Shape into a rectangle, wrap, and chill 20 minutes.

Prepare the butter block: Place the cold butter between two sheets of parchment and pound/roll into a 5x7-inch rectangle, about 1/2 inch thick. Keep edges straight. Chill while the dough rests.

Enclose the butter: On a lightly floured surface, roll the dough into a 9x13-inch rectangle. Place the butter block in the center. Fold the short ends of dough over the butter to meet in the middle, then fold shut like a book, sealing edges. You now have a butter-enclosed packet.

First turn: Roll the packet into a 8x18-inch rectangle. Fold into thirds like a letter. That’s one turn. Wrap and chill 20–30 minutes.

Second turn: Rotate the dough 90 degrees. Roll to 8x18 inches again. Fold into thirds. Wrap and chill 20–30 minutes.

Third turn: Repeat the roll and fold one more time. Wrap well and chill at least 1 hour (or up to overnight) to relax the gluten and keep layers distinct.

Shape the pastries: Roll the dough into a 12x16-inch rectangle, about 1/4 inch thick. Trim edges for neatness. Cut into 8 squares (roughly 4x4 inches). For classic pinwheels, make a 1-inch slit from each corner toward the center. Place 1 tablespoon chilled filling in the center, then fold every other corner toward the middle and press to seal. Alternatively, make rectangles and fold the long edges inward to frame the filling.

Proof: Arrange on two parchment-lined baking sheets, leaving space to puff. Brush lightly with egg wash. Cover loosely and let rise in a warm spot until slightly puffy, 30–45 minutes. They won’t double, but they should look lighter.

Bake: Preheat oven to 400°F (200°C). Brush a second coat of egg wash, sprinkle with coarse sugar if using, and bake 15–20 minutes until deep golden brown and crisp. Rotate pans halfway for even color.

Cool and glaze: Let pastries cool 10 minutes on the sheet, then move to a rack. Whisk powdered sugar, milk, vanilla, and a pinch of salt until smooth and drizzle over warm pastries.