Preheat the oven to 350°F (180°C). Grease and flour a 9x13-inch sheet pan or line it with parchment paper.
Whisk together flour, baking powder, baking soda, and salt in a large bowl.
In a separate bowl, cream together butter and sugar until light and fluffy.
Add eggs, kefir, and vanilla extract to the butter mixture, and mix until smooth.
Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Pour the batter into the prepared pan and spread evenly.
Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Let the cake cool in the pan for 10-15 minutes before transferring to a wire rack to cool completely.
Dust with powdered sugar and garnish with fresh berries or mint if desired.