In the bowl of a stand mixer fitted with the paddle attachment, beat flour, 6 tablespoons (72 grams) granulated sugar, salt, and yeast at low speed until combined. Add 6 tablespoons (84 grams) cold butter; beat until mixture resembles coarse crumbs, about 2 minutes. Add remaining 1 cup (227 grams) cold butter; pulse mixer between low speed and “o” just until butter is coated with flour. While pulsing mixer between low speed and “o,” add cold milk and ⅓ cup (80 grams) cold water in a slow, steady stream just until a shaggy dough comes together. (There will still be large pieces of butter.)
Turn out dough onto plastic wrap, and shape into a 7-inch square. Wrap in plastic wrap, and refrigerate for 1 hour.
On a lightly floured surface, roll dough into an 18×9-inch rectangle, lightly dusting work surface and top of dough with flour as needed. Pull one short side to center. Pull opposite side to center so short sides are touching. Pinch seams together; fold dough along center seam, like closing a book. Rotate dough 90 degrees; repeat rolling and folding procedure one more time. (Dough will start out soft, rough, and somewhat sticky but will come together; turn dough over as needed so edges are rolled out as evenly as possible.) Wrap in plastic wrap, and refrigerate for 30 minutes.
Repeat rolling and folding procedure one more time. Wrap in plastic wrap, and refrigerate for at least 1 hour or up to overnight.
Spray 9 wells of 2 (6-well) popover pans or 2 (6-cup) jumbo mun pans with baking spray with flour.
On a lightly floured surface, roll dough into a 203⁄4×18½-inch rectangle. Trim ¼ inch o all sides.
In a medium bowl, whisk together brown sugar, cinnamon, and remaining ¾ cup (150 grams) granulated sugar.
Using a pastry brush, brush melted butter onto dough. Sprinkle half of sugar mixture onto butter. Using a sharp knife or pizza cutter, cut dough into 27 (6×2¼-inch) rectangles.
Place 1 dough strip on work surface. Top with another dough strip, staggering from short end by about 1½ inches. Top with a third strip, staggering another 1½ inches beyond second strip. Starting with bottom strip, gently roll up strips together into a log. Place in a prepared well. Repeat stacking and rolling procedure with remaining dough. Loosely cover with a piece of greased plastic wrap. (Do not secure plastic wrap. You don’t want to put any weight or pressure on proofed dough.) Let rise in a warm, draft-free place (75°F/24°C) until very pued, 1 to 1½ hours.
Preheat oven to 425°F (220°C).
Place buns in oven; immediately reduce oven temperature to 350°F (180°C).
Bake until golden brown and an instant-read thermometer inserted in center registers 210°F (99°C), 20 to 25 minutes. Let cool enough to safely handle, up to 5 minutes.
In a shallow bowl, place remaining sugar mixture. Dredge hot buns in sugar mixture to fully coat. Serve warm, or let cool completely on a wire rack. Store unfilled buns in an airtight container for up to 3 days.
When ready to serve, spoon Maple Custard into a pastry bag, and cut a ½-inch opening in tip. Pipe about 1 tablespoon (15 grams) custard into each bun.