Preheat the oven to 180°C (350°F). Brush a bundt tin with oil then lightly dust it with flour or cocoa powder.
To make the cake: Add the flour, sugar, baking powder and salt to a large bowl. Mix until combined. Add the milk, oil and apple cider vinegar and mix until combined.
Pour half of the mixture (around 835g) into another bowl. Add the cocoa powder and milk to one bowl and the vanilla extract to the other bowl. Mix each bowls until combined.
Use 2 spoons of the same size for this step. Spoon a little of one batter into the bundt tin. Spoon an equal amount of the other batter on top. Keep alternating until there is no more cake batter.
Bake the cake in the oven for 1 hour or until a skewer can be inserted into the cake and it comes out clean. Allow the cake to cool in the bundt tin for 10 minutes then turn it upside down on and leave it on the wire rack for another 10 minutes. Gently tap the tin on the wire rack to allow it to come out.
To make the sauce: Add all ingredients to a medium-size saucepan over high heat. Bring the mixture to a boil for 5 minutes then reduce to medium heat. Simmer the mixture for 10 minutes until it is thickened. Add more sugar if you would like your sauce to be thicker. Add more cream or water if you would like your sauce to be thinner. Set aside to cool.
Drizzle the warm caramel sauce over the bundt cake and serve immediately. The cake can be stored in an airtight container at room temperature for 1 day, in the fridge for 5 days or in the freezer for up to 1 month.