Combine the semisweet chocolate and 3 tablespoons of the butter in a microwave safe bowl and heat just until melted and smooth. Set aside

In a bowl, whisk flour baking powder and salt, set aside

In a large bowl beat sugar with remaining 11/2 chose butter until light and fluffy about 4 minutes

Add eggs, one at a time beating each one in then add vanilla

Add flour mixture alternating with sour cream and stir until just combined

Remove 2 cups of batter to a medium bowl, add chocolate mixture and stir to combine

Drop batter into prepared pans in even mounds , alternating chocolate and vanilla as shown below

With a butter knife, pull it slowly through each color for a swirl as shown below. To even cake tap pan sharply on counter once or twice.

Bake 20 to 23 minutes until a Toothpick comes out just clean. Cool 15 minutes in pans then turn out onto rack to cool completely

TO FROST, place one layer, upside down on serving plate

Add 3/4 of Whipped White Chocolate Ganache Frosting on cake layer. Recipe attached below

https://cookpad.wasmer.app/us/recipes/359637-white-chocolate-ganache-whipped-frosting-filling

Place second cake layer, upside down frost with some of the Whipped Chocolate Ganache Frosting. Recipe attached below

https://cookpad.wasmer.app/us/recipes/352391-whipped-chocolate-ganache-frosting-filling

Place third, top layer upside down on last one. Frost entire cake with the Whipped Chocolate Ganache

Pipe remaining White Chocolate Ganache on top and bottom of cake. Decorate cake with sparkle sugar sprinkles