Preheat your oven to 350°F (175°C) and grease and flour a 9x5-inch loaf pan, or line it with parchment paper.

In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

In a large bowl, beat the softened butter and sugar together until light and fluffy, about 3 minutes.

Beat in the eggs one at a time, then mix in the vanilla extract until fully combined.

Gradually add the dry ingredients to the butter mixture, alternating with the milk or yogurt, mixing just until combined. Avoid overmixing.

Split the batter evenly into two bowls. Into one bowl, whisk in the cocoa powder and the extra 2 tablespoons of milk until smooth.

Spoon alternating dollops of vanilla and chocolate batter into your prepared loaf pan. Once all the batter is in, drag a butter knife or skewer through it in a gentle zig-zag or figure-eight motion to create the marbled effect. Don’t overmix.

Bake for 55-65 minutes, or until a toothpick inserted into the center comes out clean or with just a few moist crumbs.

Let the cake cool in the pan for 15 minutes, then transfer it to a wire rack to cool completely before slicing.