1. Preheat oven to 190 degrees celsius. Butter and line a 9-inch cake pan.
2. In a medium bowl, whisk together flour, baking powder and salt.
3. Using a mixer on medium speed, cream butter and sugar until fluffy for about about 2 minutes.
4. Beat in egg, egg yolk, cognac and vanilla until well-combined.
5. On low speed, add flour mixture in 3 parts, alternating with milk.
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6. Switch mixer to medium and beat for 10 to 15 seconds, just until batter appears uniform.
7. Mix coco podwer with little water to form thick paste and add into about 100g of batter, combine well. Pour into a disposable icing bag and cut a small hole on the sharp corner
8. Pour plain batter into prepared pan and smooth top with spatula or knife.
9. Squeeze the coco batter in zip Zac pattern vertically. And use a tooth pick to draw horizontal
8. Bake for about 40 minutes, or until it a skewer comes out clean.
9. Leave to rest in pan for 5 minutes before serving.