Preheat the oven to 340°F.
Line three 6-inch round cake pans.
Mix together the milk and vinegar and set aside to thicken.
Add the remaining wet ingredients to the milk mixture.
Sift in the dry ingredients and mix until just combined.
Pour the batter into the pans.
Bake for 20-30 minutes, or until a toothpick inserted into the center comes out clean.
Let the cakes cool before leveling them.
Using a hand mixer or stand mixer with a paddle attachment, cream the butter on medium-high until creamy and light. With the mixer on low, gradually add the powdered sugar, scraping down the sides of the bowl after each addition. Add the cocoa powder and milk, and mix until well combined.
In a medium saucepan, melt the butter over medium heat, then add the brown sugar. Mix until dissolved. Add the coconut milk and cornstarch mixture, and stir for 5 minutes until it bubbles and looks like thin caramel. It will thicken as it cools. Remove from heat and fold in the chopped pecans and coconut flakes. Let cool.
Layer the cakes with the frosting and coconut pecan caramel filling.