Preheat oven to 350° and line 9x13 baking pan with foil.
Start melting butter in a sauce pot over medium heat. Take off heat once melted and pour it into a mixing bowl. Let it cool for about 5 minutes.
Add sugar once butter is slightly cooled and mix well.
Whisk in eggs one at a time, whisking well after each addition.
Whisk in cocoa powder, food coloring, vanilla extract, and vinegar.
Add flour, baking powder, and salt. Whisk until everything is completely incorporated.
Pour batter into the prepared baking pan and make sure it's spread evenly throughout.
Bake for 22-27 minutes. Do a toothpick test to make sure the brownies are done. (Remember it's better to slightly under-bake then over-bake brownies or they will be dry.)
Let brownies cool completely before frosting.
Frosting:
Beat butter and cream cheese together, on medium-high speed, until smooth, 1-2 minutes. Scrape sides and bottom of the bowl and beat for a few more seconds.
Lower the speed to low and beat in powdered sugar. Turn up the speed back to medium-high and beat for about 2 minutes, until the frosting is light, smooth and fluffy.
Melt white chocolate in a microwave at half power for 30 seconds, stir and do another 30 seconds, still on half power. (Repeat another 30 seconds if needed.) When chocolate is almost all melted, just keep stirring until it's all melted.
On low speed, drizzle in melted white chocolate and mix well, make sure to scrape sides and bottom of the bowl and mix until all even.
Frost cooled brownies evenly all over the top.
To decorate with shaved chocolate, I use a large (thicker) cheese grater to make the curls. Slowly run the side of white chocolate bar over the cheese grater to make little curls. (Make sure chocolate is not cold but at room temperature.)
Sprinkle chocolate curls all over the top of frosting.