✓ Preheat the oven to 350°F / 175°C and position the rack in the middle

✓ Grease a 9×13-inch sheet pan with butter and line with parchment paper

✓ Whisk together flour, baking powder, baking soda, and salt in a large bowl

✓ Beat the eggs with sugar until the mixture lightens in color and volume

✓ Stir in melted butter, kefir, vanilla extract, and lemon zest until just incorporated

✓ Gradually add the dry ingredients to the wet mixture, folding until no streaks remain

✓ Pour the batter into the prepared pan and spread to an even layer

✓ Bake for 25–30 minutes, or until the top is golden brown and a toothpick emerges clean

✓ If edges brown too quickly, tent the pan with foil after 20 minutes

✓ Remove the cake and rest in the pan for 10 minutes

✓ Run a knife around the perimeter, invert onto a cooling rack, and peel off parchment

✓ Cool completely before slicing

✓ Dust with powdered sugar or drizzle a kefir glaze

✓ Cut into 24 squares for standard servings