**Preheat oven** to 350°F (175°C). Grease a 9x13-inch sheet pan or line with parchment paper.
In a **large bowl**, sift together flour, baking powder, baking soda, and salt.
In another bowl, **whisk eggs** until blended. Add sugar and whisk until light and frothy.
**Add kefir**, then slowly whisk in melted butter and vanilla extract until smooth.
Gradually **combine wet and dry ingredients**, mixing just until no streaks remain. Do not overmix.
Fold in any **optional add-ins** like nuts, chocolate chips, or fruit.
Pour batter into the prepared pan and **spread evenly**.
Bake for **28–32 minutes**, or until a toothpick inserted in the center comes out clean.
Let the cake **cool in the pan** for 10 minutes, then transfer to a wire rack to cool completely before frosting.