Prep Your Baking Pan & Oven
Preheat your oven to 350°F (175°C).
Grease a 9x13-inch sheet pan with butter or non-stick spray, or line it with parchment paper for easy removal.
Mix the Dry Ingredients
In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, and salt until well combined.
If using any add-ins like cinnamon, chocolate chips, or citrus zest, stir them into the dry ingredients now for even distribution.
Prepare the Wet Ingredients
In a separate bowl, whisk together the kefir, melted butter, eggs, and vanilla extract until smooth and fully combined.
Make sure your butter has cooled slightly before adding it to prevent cooking the eggs.
Combine the Batter
Gradually add the wet ingredients into the dry ingredients, stirring gently with a spatula or whisk.
Mix until just combined—avoid overmixing, as this can make the cake dense instead of light and fluffy.
Bake to Perfection
Pour the batter evenly into the prepared sheet pan, smoothing the top with a spatula.
Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
The cake should be golden brown and slightly springy to the touch.
Cool & Serve
Let the cake cool in the pan for about 10 minutes, then transfer it to a wire rack to cool completely.
Enjoy it plain, dusted with powdered sugar, or topped with your favorite frosting!