Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.

In a separate large bowl, combine the kefir, sugar, eggs, melted butter, and vanilla extract. Mix until well blended.

Gradually add the dry ingredients to the wet ingredients, mixing just until incorporated. Be careful not to overmix.

Pour the batter into the prepared baking pan and spread it evenly.

Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.

Allow the kefir sheet cake to cool completely before frosting and serving.