Whisk together the flour, baking powder, and salt in a medium bowl. In a large bowl, beat the softened butter and granulated sugar for 2 to 3 minutes, until smooth and slightly lighter, then beat in the egg, egg yolk, and vanilla until fully combined. Add the dry ingredients and mix just until a soft dough forms with no visible dry flour, stopping as soon as it comes together, since overmixing makes the cookies tougher and harder to roll neatly.

Divide the dough into 2 equal portions, flatten each into a rough rectangle, wrap, and chill for 20 to 30 minutes. The dough should be cool and firmer but still pliable; too soft and the filling will smear, too cold and it may crack while rolling.

Mix the soft butter, brown sugar, cinnamon, flour, and salt in a small bowl into a spreadable paste. The flour here keeps the filling from melting out too aggressively during baking.

Working with one dough portion at a time on a lightly floured surface, roll into a rectangle about 20 x 25 cm and 4 to 5 mm thick. Spread half of the cinnamon filling evenly over the surface, leaving a 1 cm border on one long edge, then roll the dough up tightly from the opposite long edge into a log and pinch the seam gently closed. Repeat with the second portion. Roll tightly to keep the spiral defined, avoid too much bench flour or the dough won't seal well, and chill briefly if the dough softens or the filling starts slipping while rolling.

Wrap each log tightly in parchment or plastic wrap and chill for at least 1 hour, or until firm enough to slice cleanly. Rotating the logs once halfway through chilling keeps them from flattening on one side, for extra-clean spirals. Don't skip this chill; firm logs are what keep the cookies thick, the swirls clean, and the filling from leaking out.

Heat the oven to 175°C / 350°F and line baking trays with parchment. Trim the ends if uneven, then slice each log into 12 cookies, about 1.5 cm thick, for 24 total. Gently round any slice that loses its shape with your fingers before placing it on the tray.

Arrange the cookies with space between them, about 6 to 8 per tray, and bake for 12 to 14 minutes, until the edges are lightly golden, the centers are set but still look slightly soft, and the spiral lines look a little glossy and caramelized. Cool on the tray for 5 minutes before transferring to a rack. Don't overbake; these cookies continue to set as they cool, so pull them when the centers are just set for a thicker, slightly gooey swirl.

Whisk the powdered sugar, 2 tablespoons of milk or cream, vanilla, and salt until smooth, adding a little more liquid only if needed. The glaze should be fluid enough to drizzle but not so thin that it floods the spiral. Drizzle lightly over the cooled or just-warm cookies, keeping the spiral clearly visible underneath.