Preheat the oven: Preheat your oven to 350°F (175°C). Grease and flour a 9×13-inch baking pan or line it with parchment paper.

Mix dry ingredients: In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.

Prepare wet ingredients: In a large mixing bowl, cream the softened butter and sugar together until light and fluffy, about 2-3 minutes. Add the eggs, one at a time, beating well after each addition. Stir in the vanilla extract and lemon zest (if using).

Add kefir: Gradually add the kefir to the butter mixture, mixing until fully combined.

Combine: Slowly add the dry ingredients to the wet ingredients, mixing gently until just incorporated. Be careful not to overmix, as that can lead to a denser cake.

Bake: Pour the batter into the prepared baking pan and smooth the top with a spatula. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.

Cool: Allow the cake to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.

Glaze or frost: If desired, whisk together the powdered sugar, kefir, and vanilla extract in a small bowl. Drizzle the glaze over the cooled cake for a tangy finish. Alternatively, you can frost it with whipped cream or cream cheese frosting for a richer taste.

Serve: Slice into squares and serve. Enjoy!