Divide the batter in two. I like to weigh the total batter and divide it exactly into two parts. To one of the halves, add the ingredients for the red velvet marbling and mix well to incorporate.
Line a 22x11cm/8x4in with baking paper.
Place the vanilla batter in a piping bag with a round tip of about 1cm/⅓in in diameter. Form a thick vanilla batter line along the centre of the base of the pan. Exactly on top of that one, pipe a red velvet batter line. On top of that one, make a vanilla line and go on like this until you have used up all the batter. It's very important that you make the line exactly on top of the other to get the zebra effect. Alternatively, you can also do a classic marbling by adding bits of each colour of batter in the pan and then using a knife or skewer to marble. Tap the pan against your surface to level the batter.
Optional: cut thin strips of cold butter and place them along the centre of the batter. This will make it open evenly at the centre.
Bake the cake in a preheated oven at 180°C/350°F for 45 min - 1 hour or until a serrated knife comes out clean from the middle. It's important not to overbake it so it doesn't dry up. If you have an electric oven, check the cake at the 30-minute mark to make sure it's not going overly golden on top. If it's golden enough, switch to only bottom heat or place aluminium foil on top.
Release the cake from the pan while it's still hot.