Preheat the oven to 375 F (190 C). Line two large baking sheets with parchment paper.
In a large bowl, whisk the flour, baking soda and salt together until well combined.
2¼ cups (270 g) all purpose flour, ½ teaspoon baking soda, ¼ teaspoon salt
In another large bowl, cream the room temperature butter, light brown sugar and granulated sugar until well combined. On low, beat in the egg and vanilla extract until smooth and well combined.
½ cup (113 g) unsalted butter, room temperature, ¾ cup (150 g) granulated sugar, ¼ cup (50 g) light brown sugar, 1 large egg (US), room temperature, 1½ teaspoons (8 ml) vanilla extract
Mix the dry ingredients into the wet, alternating with the buttermilk in this order: dry, buttermilk, dry, buttermilk, dry until just combined.
½ cup (118 ml) buttermilk
Scoop about 1 tablespoon (15 ml) of buttermilk sugar cookie dough onto a the prepared baking sheet. I like to use a small cookie scoop to get perfectly portioned cookies. Bake the cookies for 10 - 12 minutes. The edges will be set. The center will bounce back when gently pressed.
Remove the cookies from the oven onto a cooling rack or stove top. After 3 minutes, transfer the cookies off the hot baking sheet and onto the cooling rack to cool to room temperature. While the cookies cool, make the buttermilk glaze.
Whisk the buttermilk and vanilla extract into the powered sugar until smooth and well combined. For a thinner glaze, add more buttermilk. For a thicker glaze add more powdered sugar. Top each soft buttermilk cookie with the glaze and enjoy! 1½ - 2 cups (180 - 240 g) powdered sugar, 3 - 4 tablespoons (45 - 60 ml) buttermilk, 1 teaspoon (5 ml) vanilla extract