Prepare for Baking:

Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan or line it with parchment paper.

Mix the Dry Ingredients:

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

Blend the Wet Ingredients:

Beat the eggs and sugar until light and fluffy. Gradually add melted butter, followed by kefir and vanilla extract. Mix until smooth.

Combine Batter:

Slowly fold the dry ingredients into the wet mixture in batches. Add any optional add-ins, like lemon zest or berries, at this stage.

Bake:

Pour the batter into the prepared pan, spreading it evenly. Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean.

Cool:

Let the cake cool in the pan for 10 minutes before transferring it to a wire rack. Allow it to cool completely before adding any toppings.