1. Preheat the oven to 350F/180C and heavily grease a bundt pan, I used this Nordicware Brilliance Pan that needs to be greased in all the nooks and crannies otherwise the cake will stick. I like to use Baker's Joy or Baking Spray.
2. In a medium bowl combine the flour, baking powder, and salt. Set aside.
3. In a bowl of an electric mixer fitted with a paddle attachment or a large bowl with a handheld electric mixer on medium speed combine the butter and sugar for 4-5 minutes, so it's nice and fluffy. Add in the vanilla extract then eggs one at a time, making sure to scrape the sides of the bowl.
4. In a small bowl mix sour cream and buttermilk.
5. To the butter mixture add 1/3 of the dry ingredients, mixing until just combined, then 1/2 of the wet ingredients, then repeat with the remaining ingredients ending with the dry ingredients.
6. Scoop two cups of cake batter into another medium bowl and add in the cocoa powder, melted chocolate and milk then mix until just combined.
7. I used a large ice cream scoop to add one scoop of vanilla cake batter, then a scoop of chocolate, and alternating until full. You can then use a butter knife to gently swirl the cake, sometimes I do this, other times I don't. Bake for 40-50 minutes or until a toothpick inserted in the center of the cake comes out clean. Depending on the bundt cake pan you use the bake time will need to be adjusted, a wider bundt cake will cook quicker than a deeper bundt cake. Let the marble cake cool on a wire rack for 30 minutes then I use a small angled spatula to gently release the cake from the pan then place a wire rack on top of the cake and flip over to invert the cake onto the cooling rack.  

Chocolate Frosting

1. In a small saucepan add the cream and cook over medium heat for a couple of minutes to warm it, you don't want to boil it though.
2. In a large bowl add the chocolate and corn syrup then pour the warmed cream directly over it. Let it sit untouched for 5 minutes then using a whisk mix well together until a shiny homogenous chocolate ganache is formed. Store in the freezer for 20 minutes, I stir every 5 minutes or so, to chill the ganache and thicken it a bit.
3. When ready to use I like to pour it into a measuring cup with a pour spout to easily drizzle it on the cake. Store the cake in an air-tight container at room temperature for up to 3 days. If you are in a very warm environment I would store it in the fridge but let sit at room temperature for 30-60 minutes before serving.