Make the cake: Preheat the oven to 350°F. Lightly grease three 6" cake pans and line with parchment paper.
In the bowl of a stand mixer, whisk together the sugar, flour, baking powder, and salt. Add the butter and beat with the paddle attachment at low speed, until the mixture looks sandy.
Combine the milk and vanilla and add all at once. Mix at low speed for 30 seconds, then increase the speed to medium and beat for 30 seconds.
Scrape the bottom and sides of the mixing bowl.
With the mixer running at low speed, add 1 egg. Increase the speed to medium and beat for 30 seconds.
Repeat this procedure with the second egg. Continue adding the eggs, scraping after each addition, until all 4 are added.
After the last egg is added, scrape the bowl once more, then beat at medium-high speed for 30 more seconds.
Transfer the batter to each pan, smooth the tops. and smack the pans on the counter to get the air bubbles out of the batter.
Bake for 24-27 minutes or until the cake is golden brown around the edges and just beginning to pull away from the edge of the pan. A toothpick inserted in the center will come out clean.
Remove the cake from the oven and allow it to cool for 5 minutes, then turn it out onto a rack to cool completely.
Make the frosting: In a bowl of stand mixer fitted with paddle attachment, beat the butter on medium speed until pale and fluffy, about 6 minutes.
Sift together the powdered sugar and cocoa powder. On low speed, add the powdered sugar, cocoa powder, water, sour cream, vanilla, and salt and beat until incorporated.
Increase the speed to medium and beat for 2 more minutes. Add the melted chocolate and beat on medium speed until smooth. Add 1 tsp of the black gel color and mix until combined. Wait about 15 minutes for the color to develop fully. Add more color if necessary until you get the desired tint.