Preheat the Oven: Preheat your oven to 350°F (175°C). Grease and flour a 9x13-inch baking dish or line it with parchment paper.

Combine Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.

Cream Butter and Sugar: In a large bowl, cream the softened butter and granulated sugar until light and fluffy, about 2-3 minutes.

Add Eggs and Vanilla: Beat in the eggs one at a time, ensuring each egg is well incorporated before adding the next. Stir in the vanilla extract.

Add Dry Ingredients and Kefir: Gradually add the dry ingredients to the butter mixture, alternating with the kefir. Begin and end with the dry ingredients. Mix just until combined and the batter is smooth.

Bake the Cake: Pour the batter into the prepared baking dish and smooth the top with a spatula. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.

Cool the Cake: Allow the cake to cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely.

Cream the Butter: In a large bowl, cream the softened butter until smooth and creamy.

Add Powdered Sugar: Gradually add the powdered sugar to the butter, mixing well until the frosting is smooth and creamy.

Add Kefir and Vanilla: Stir in the kefir, vanilla extract, and a pinch of salt. Mix until the frosting is light and fluffy.

Frost the Cake: Once the cake is completely cooled, spread the frosting evenly over the top.

Serve: Slice the cake into squares and serve. Enjoy!