Set an oven rack in the center and heat the oven to 175°C conventional or 165°C fan. Line one light-colored rimmed baking sheet with parchment paper. In a small bowl, whisk the 25 g rolling sugar with the cinnamon until no dark clumps remain. Preparing the coating now prevents the portioned dough from warming while you search for ingredients later.

Place the softened butter and 65 g granulated sugar in a medium bowl. Using a sturdy silicone spatula, press and beat them together for 60 to 90 seconds, scraping the bowl several times, until the mixture is creamy, slightly aerated, and visibly lighter. Add the egg yolk and vanilla, then mix for about 30 seconds until smooth and fully emulsified, with no slippery streaks of yolk.

In a small bowl, combine the flour, cream of tartar, baking soda, and salt with a whisk or fork. Add the dry ingredients to the butter mixture and fold just until no dry flour remains, scraping the bottom of the bowl. The dough should be soft but cohesive. Cover the bowl and refrigerate for 20 minutes, until the dough feels cool and rolls cleanly without appearing oily.

Divide the chilled dough into eight equal portions of about 29 to 30 g each, using a scale or a level tablespoon cookie scoop. Roll each portion gently between your palms into a smooth ball, then coat it thoroughly in the cinnamon sugar. Arrange the balls at least 5 cm apart on the prepared baking sheet and press each one lightly to about 1.5 cm thick for even spreading.

Bake on the center rack for 9 to 11 minutes, rotating the pan once after 7 minutes if your oven browns unevenly. The edges should look set, the tops should be puffed and crackled, and the centers should appear matte rather than wet, although they will remain soft. Cool on the pan for 10 minutes, then transfer carefully to a wire rack and cool for another 5 minutes before serving.