1. Gather equipment digital scale electric hand mixer rubber spatula loaf pan digital oven thermometer infrared thermometer gun cooling rack Preheat oven to 347 F and line a loaf pan with parchment paper or butter and flour it

2. Bring butter eggs and yogurt to room temperature Use unsalted butter that is soft but not oily and use plain whole milk yogurt such as Chobani or Fage

3. Sift flours baking powders salt and cocoa separately to remove lumps and aerate the dry ingredients

4. Beat butter and sugar with an electric hand mixer until pale and fluffy This takes a few minutes and adds air for a light crumb

5. Add eggs one at a time while the mixer runs on low to medium speed Mix until each egg is incorporated before adding the next

6. Mix in yogurt and alcohol free vanilla flavor then stop the mixer and switch to a rubber spatula to fold in dry ingredients

7. Divide the wet batter into two equal portions Into the first portion gently fold the vanilla dry mixture until no streaks remain

8. Into the second portion gently fold the chocolate dry mixture until evenly combined Keep folding motions light to avoid overworking the batter

9. Transfer each batter to a piping bag or use spoons Alternate piping or spooning vanilla and chocolate batters into the prepared loaf pan to create layers

10. Drag a knife through the batter in a single zigzag pattern to form the marble effect Avoid over mixing or the colors will blend into one tone

11. Smooth a very thin line of softened butter across the top of the batter to encourage an even rise and a bakery style split

12. Bake for about 45 minutes or until a skewer inserted in the center comes out clean Use a digital oven thermometer to confirm oven accuracy

13. While the loaf bakes make the syrup by heating water and sugar until the sugar dissolves and the mixture slightly thickens

14. When the loaf is done pour the warm syrup over it in stages allowing it to sink in Let the loaf cool in the pan briefly then transfer to a cooling rack

15. Chill the loaf for one hour in the refrigerator or fifteen minutes in the freezer before glazing if using a glaze

16. To make the glaze place high quality chocolate in a bowl and melt slowly Stir every 30 seconds in the microwave or use a bain marie Mix in vegetable oil when smooth then cool to about 77F before using

17. Stir chopped roasted hazelnuts into the glaze if desired Pour the glaze over the chilled loaf and return to the refrigerator briefly to set

18. Store the loaf in an airtight container at room temperature for 2 to 3 days or refrigerate if your kitchen is warm