Preheat the oven: Set it to 340°F and line a large baking sheet with parchment paper.
Mix the wet ingredients: In a bowl, whisk the egg, salt, sugar, and vanilla extract. Stir in the cooled melted butter, then add kefir.
1 egg, ¼ cup white sugar, ½ teaspoon salt, ⅔ cup kefir, 1 teaspoon vanilla extract, ¼ cup unsalted butter
Combine the dry ingredients: In a separate bowl, mix the whole wheat flour, baking powder, and cinnamon.
2 cups whole wheat flour + more for dusting, 1 teaspoon baking powder, 1 teaspoon ground cinnamon
Incorporate the dry into the wet: Gradually add the dry ingredients to the wet mixture. Use a spatula at first, then knead lightly on a floured surface until a soft dough forms. Avoid overmixing to keep the cookies tender.
Shape the cookies: Take 1 tablespoon of dough, roll into a ball, and coat in sugar if desired.
3 tablespoons white sugar for rolling
Arrange and flatten: Place on the baking sheet, pressing down gently.
Bake: For 20 minutes for soft cookies, or 25 minutes for a slightly crisp top.
Cool: Let cool on the baking sheet for 5 minutes before transferring to a wire rack.