Begin by preheating your oven to 350°F (175°C). While the oven heats, line two baking sheets with parchment paper to prevent sticking.
In a large mixing bowl, use an electric mixer to beat together the softened butter, granulated sugar, and packed brown sugar. Mix until the mixture is light and fluffy, which should take about 3-4 minutes.
Add the eggs one at a time to the butter mixture, ensuring each egg is fully incorporated before adding the next. Follow with the pure vanilla extract and mix well.
In a separate medium bowl, whisk together the all-purpose flour, baking powder, salt, and ground cinnamon. This will help to evenly distribute the leavening agent and spices.
Gradually mix the dry ingredients into the butter and sugar mixture. Stir until just combined; avoid overmixing to maintain a tender cookie texture.
Gently fold in the chopped walnuts (or pecans) and sour cream until the batter is smooth and homogenous.
Using a cookie scoop or a heaping tablespoon, drop rounded mounds of dough onto the prepared baking sheets, spacing them about 2 inches apart to allow for spreading.
Bake the cookies in the preheated oven for 12-15 minutes, or until the edges turn a light golden brown. Once baked, remove them from the oven and let them cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.
For the glaze, whisk together the powdered sugar and milk in a small bowl until smooth. Once the cookies have cooled, drizzle the glaze over the top of each cookie with a spoon or a piping bag for a more decorative finish.
Notes
For an appealing touch, sprinkle a light dusting of additional ground cinnamon or powdered sugar on top.