Preheat your oven to 350°F (175°C) and line a 9×13-inch baking pan with parchment paper.
Whisk together flour, baking soda, and salt in a bowl.
In another bowl, cream the butter and sugar until light and fluffy.
Add eggs one at a time, beating well after each addition.
Mix in kefir and vanilla extract until smooth.
Gradually add the dry ingredients and stir gently to combine.
Pour the batter into the prepared pan and spread evenly.
Bake for 30–35 minutes or until a toothpick inserted in the center comes out clean.
Cool on a wire rack, then dust with powdered sugar if desired.