Step 1: Preheat your oven to 350 °F (175 °C). Line two baking sheets with parchment paper.
Step 2: In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
Step 3: In a large bowl or the bowl of a stand mixer fitted with the paddle attachment, beat the softened ¾ cup unsalted butter and granulated sugar on medium speed for 2–3 minutes until pale and fluffy.
Step 4: Beat in the large egg until fully incorporated, scraping down the sides of the bowl as needed. Then add the vanilla extract and mix briefly.
Step 5: With the mixer on low speed, gradually add the dry ingredients in two additions, alternating with the 2 tablespoons of milk or buttermilk. Mix just until no streaks of flour remain. Do not overmix.
Step 6: In a separate small bowl, combine the ½ cup packed brown sugar, 1 tablespoon ground cinnamon, and 2 tablespoons melted unsalted butter. Stir until the mixture is crumbly but holds together when pressed.
Step 7: Lightly flour your workspace and turn the dough out onto it. Roll the dough into a 12x8-inch rectangle, about ¼ inch thick, keeping the edges as straight as possible.
Step 8: Evenly sprinkle the cinnamon swirl mixture over the entire dough rectangle. Gently press it in with your hands or the rolling pin to ensure it adheres.
Step 9: Starting at one of the long edges, carefully lift the dough and roll it away from you, keeping the roll as tight as possible. Pinch the seam to seal it closed and gently adjust the ends to form a neat 12-inch log.
Step 10: Using a sharp knife or unflavored dental floss, cut the log crosswise into ½-inch-thick rounds, yielding approximately 24 cookies.
Step 11: Place the cookie rounds cut-side up on your prepared baking sheets, spacing them about 1 inch apart. Bake in the preheated 350 °F (175 °C) oven for 10–12 minutes, or until the edges just begin to turn light golden. The centers should still look soft.
Step 12: While the cookies are baking, prepare the optional glaze. In a medium bowl, whisk together the 1 cup powdered sugar, 2 tablespoons of milk or cream, and ¼ teaspoon vanilla extract. Adjust consistency as needed.
Step 13: Once the cookies are out of the oven, let them rest on the baking sheet for 2 minutes. Then, drizzle the glaze over each warm cookie or spread it with the back of a spoon. Transfer to a wire rack to finish cooling and allow the glaze to set.