Preheat the oven to 180C/160C Fan. Line a muffin tray with your muffin liners and leave to one side.
Add the oil, whole milk, eggs and vanilla to a large bowl or jug and whisk until they are all combined. Leave to one side.
In a separate bowl add the flour and sugar and mix until fully combined.
Add the wet ingredients to the dry ingredients and whisk until the mixture is smooth and combined.
Fold through the fudge pieces until they are evenly distributed.
Split the mixture evenly into two bowls.
Add the cocoa powder to one bowl and mix until completely incorporated.
Add a teaspoon of chocolate mixture into the bottom of each case and then add in the caramel mixture. Repeat until you have used up all the muffin batter.
Now using a skewer gently swirl the two batters together. We don't want to mix them completely just to swirl the two batters around to get a lovely marble effect.
Bake in the oven for 25-30 minutes. Once they have baked leave them to cool fully in the muffin tray.