Add the softened butter to a mixing bowl.

Add the eggs and mix well.

Add the sugar and mix until combined.

Add the yogurt and mix until smooth.

In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.

Gradually add the dry ingredients to the wet mixture and mix until combined.

Add the buttermilk and mix until a smooth batter forms.

Transfer a portion of the batter to a separate bowl and mix in the cocoa powder with the remaining 1 tablespoon of buttermilk.

Pour the vanilla and chocolate batters alternately into a lined loaf pan and gently swirl to create a marble effect.

Bake at 350°F (175°C) for 50–60 minutes, or until a toothpick inserted into the center comes out clean.

Let the cake cool completely before slicing and serving.