Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking pan and set it aside.
Prepare the dry ingredients: In a large mixing bowl, whisk together the flour, sugar, baking soda, baking powder, and salt.
Mix the wet ingredients: In another bowl, whisk the eggs, kefir, melted butter, vegetable oil, and vanilla extract until smooth.
Combine wet and dry ingredients: Gradually add the wet ingredients into the dry ingredients, stirring gently until just combined. Do not overmix.
Fold in any optional add-ins (such as zest, cinnamon, or chocolate chips) if using.
Bake the cake: Pour the batter into the prepared pan and spread it evenly. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.
Cool the cake: Allow the cake to cool in the pan for about 10 minutes, then transfer it to a wire rack to cool completely.
Prepare the glaze (optional): In a small bowl, whisk together powdered sugar, milk (or kefir), and vanilla extract. Drizzle over the cooled cake.
Serve and enjoy: Slice the cake into squares and serve. Store leftovers in an airtight container at room temperature for up to 3 days or refrigerate for longer freshness.