Preheat the oven to 180°C (350°F) and line a 12-cup muffin tin with paper liners.
In a large bowl, whisk together the flour, baking powder, baking soda, salt, and granulated sugar.
In another bowl, beat the eggs, then add the yogurt, vegetable oil, and vanilla extract. Mix until smooth.
Pour the wet ingredients into the dry ingredients and gently mix until just combined. Do not overmix.
Divide the batter into two equal portions.
Add the cocoa powder to one portion and gently fold it in until fully incorporated.
Using a spoon, alternate spoonfuls of vanilla and cocoa batter into each muffin cup to create a bicolored effect.
Using a toothpick or skewer, gently swirl the two batters together in each cup to create a marbled pattern.
Bake for 18-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and let them cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.