In a bowl, mix the soy milk and vinegar together and whisk well. Let the mixture sit for 5 minutes. In a separate bowl, combine the flour, sugar, vanilla sugar, and baking powder.
After 5 minutes, add the oil and yogurt to the soy milk and vinegar mixture. Whisk until combined. Then, add the wet ingredients to the dry ingredients and gently mix until everything is just combined. Be careful not to overmix the batter, or it will get dense.
Divide the batter evenly between two separate bowls. In one bowl, add the lemon zest and lemon juice. In the other bowl, add the cocoa powder and 2 tablespoons of soy milk. Mix each batter until well combined.
Either line a 12-cup muffin tray with paper liners or skip them if you're using stainless steel like me! Add about 1 tablespoon of the light batter to each muffin cup, followed by about 1 tablespoon of the cocoa batter. Use a fork or skewer to gently swirl the two batters together.
Bake at 180°C (350°F) for about 20 to 25 minutes, or until a toothpick inserted into the center comes out clean. Let the muffins cool before enjoying.
