Prepare your Loaf pan Whisk egg in a separate bowl then slowly mix in while your Electric hand mixer is on.
Preheat oven to 175 C / 347 F
In a bowl beat sugar with room temperature, soft butter with an Electric hand mixer (or stand mixer) for few minutes until fluffy
Mix in egg, small amount at a time, and whip the mixture for another minute
Add yogurt in and mix in vanilla
Sift the dry ingredients for the vanilla part together (flour, baking powder, salt)
Sift the dry ingredients for the chocolate part together into a different bowl (flour, cocoa powder, baking powder, salt)
Divide the wet ingredients into two and Prepare the vanilla cake butter by gently folding the flour, baking powder, salt mixture into one of the wet ingredients.
Prepare the chocolate batter by gently folding the flour, cocoa powder, baking powder, salt mixture into the other wet ingredients.
Fill a piping bag with your chocolate cake batter, another one with your vanilla cake batter and pipe the cake batters into your prepared Loaf tin by alternating between the vanilla and chocolate cake batter
To create the marble effect grab a knife and make some zigzag movement in the pan. Make sure that you do not over mix the cake batters, or they might melt together
On top of the cake apply a very thin butter line (with the help of a pastry bag)– this will help the Loaf cake to open up nicely and evenly
Bake for 45 min or until skewer inserted comes out clean
Towards the end of the baking time make the syrup: boil water and sugar in a saucepan until slightly thickens and reaches syrup consistency
Once loaf is baked, pour the warm syrup over in a few stages, this is the drizzle stage. The syrup will sink in. Let the cake cool few minutes in the tin and move it onto a cooling rack.
Chill the cake 1h in the fridge before glazing (or 15 minutes in the freezer)