Mix 60g butter, 90g brown sugar, 1 tbsp cinnamon, salt, and vanilla until a thick paste forms
Roll cinnamon mixture into small balls (5–7 g each)
Freeze cinnamon balls for 30 minutes until firm
Cream 1 cup butter and sugars until light and fluffy
Add egg, egg yolk, and 1 tbsp vanilla to butter mixture and mix until combined
Mix in baking soda, salt, and cream or yogurt
Add flour and mix just until incorporated
Gently fold in frozen cinnamon swirl balls using only 3–4 folds to keep a marbled effect
Scoop dough into balls of 70–90 g each
Refrigerate dough balls overnight
Let dough balls sit at room temperature for 10–15 minutes before baking
Preheat oven to 180°C / 350°F
Bake cookies for 8–9 minutes
Open oven and gently tap baking tray to help cookies spread
Return to oven and bake 3–4 more minutes (11–13 minutes total)
Let cookies cool on tray for 10 minutes before transferring
Mix powdered sugar with 1 tbsp milk for glaze
Add more milk drop by drop until thick, paste-like consistency
Add extra powdered sugar for firmer glaze if desired
Transfer glaze to piping bag and pipe lines over cooled cookies
