Preheat Oven:

Preheat your oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan or line it with parchment paper.

Mix Dry Ingredients:

In a large mixing bowl, sift together the all-purpose flour, granulated sugar, unsweetened cocoa powder, baking powder, baking soda, and salt.

Combine Wet Ingredients:

In another bowl, whisk together the eggs, plain kefir, warm water, vegetable oil, and vanilla extract until well combined.

Combine Wet and Dry Mixtures:

Pour the wet ingredients into the dry ingredients. Stir until the batter is smooth and free of lumps.

Pour Batter into Pan:

Transfer the batter into the prepared baking pan, smoothing the top with a spatula.

Bake:

Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center comes out clean.

Cool:

Allow the cake to cool completely in the pan on a wire rack before slicing and serving.

Optional Frosting:

For an extra layer of indulgence, top the cooled cake with a chocolate ganache or your favorite chocolate frosting.