Grease and line a 1/2 kg (or 1 pound) loaf pan and preheat your oven to 175C.
In a medium-large bowl add your butter, cream cheese and sugar, whisk until combined. Now pour in your condensed milk and whisk for 30 seconds.
In the same bowl, sift your flour, baking powder and salt. Gently mix.
Now add the milk & vanilla extract to this batter.
Gently whisk until the batter just comes together. Do not over mix.
The red velvet portion:
Pour half of this batter into another clean bowl and add the yoghurt, sifted cocoa powder and red gel food colour in one bowl. Gently fold until just combined.
If the vanilla batter is too thick, you can add one or two tablespoon of milk and fold it in to combine,
REMEMBER, do not over mix your batter.
Using a spoon, add portions of the cake batter, alternating between the vanilla and red velvet cake batter till you run out of it. Take a knife or a skewer and gently swirl it to your liking, do not swirl too much or else the batters will end up mixing. Alternatively you can also fill the batter into two piping bags and pipe dollops of it alternately and swirl.
Bake in a preheated oven for approx 35 minutes or until a toothpick inserted into the center of the cake comes out clean. Depending on the temperature of your oven and the type of pan you use, the baking time can vary a few minutes so check after 25 minutes.
Let the cake cool in the pan before removing and slicing.