Prepare the Baking Sheets: Line two cookie sheets with parchment paper. Set aside.
Make the Cookie Dough: In a large bowl, beat the butter (1/2 cup) and sugar (3/4 cup) together with a hand mixer on medium speed until light and creamy, about 2 minutes. Add the egg (1) and vanilla (1 1/2 teaspoons), and continue mixing until fully combined and slightly fluffy, another 1–2 minutes.
Combine Dry Ingredients: In a separate bowl, whisk together flour (1 3/4 cups), baking powder (1/2 teaspoon), and salt (1/4 teaspoon). Add the dry mixture to the butter mixture along with the sour cream (2 tablespoons), mixing just until no streaks of flour remain. The dough should be soft and slightly tacky but not sticky.
Chill the Dough: Shape the dough into a thick disc, wrap it tightly in plastic wrap, and refrigerate for at least 1 hour (or up to overnight). Chilling helps the dough firm up for clean spirals.
Preheat + Roll: Preheat oven to 350°F (177ºC). Lightly flour your counter and rolling pin. Roll the chilled dough into a 10x12-inch rectangle, about ¼ inch thick.
Cinnamon Filling
Add the Cinnamon Filling: In a small bowl, combine brown sugar (1/4 cup) and cinnamon (1 tablespoon). Sprinkle evenly over the dough, pressing lightly so it adheres.
Roll + Slice: Starting from a long edge, roll the dough tightly into a log. Pinch the seam closed and gently roll to even out the thickness. Slice the log into ½-inch rounds using a sharp knife or unflavored dental floss (for the cleanest edges). Place cookies about 2 inches apart on the prepared sheets and gently flatten each spiral slightly.
Bake: Bake for 10–12 minutes, or until the edges are just golden and the centers look set. Let cookies cool on the baking sheet for 2 minutes before transferring to a wire rack to cool completely.
Glaze
Glaze: In a small bowl, whisk together powdered sugar (1 cup), heavy cream (2 tablespoons), vanilla (1 teaspoon), and a pinch of salt until smooth and pourable. Adjust consistency with more cream or sugar as needed. Drizzle the glaze over cooled cookies. Let it set before serving.