Preheat oven to 350 F. Grease 2 9-inch round cake pans then line with parchment paper circles. The grease will help hold the parchment in place as well as help you remove the cake when done.
In a medium bowl, stir or whisk together the cake flour, baking powder and salt. Set aside.
Add extracts to the milk in measuring cup or bowl. Set aside.
With mixer on medium speed, beat butter until creamy. Gradually add in sugar until incorporated. Add eggs and yolk one at a time until incorporated. Turn off mixer and scrape sides of bowl.
With mixer on low speed, gradually add half of the dry ingredients, then half of the milk mixture. Mix in remaining dry and wet ingredients and mix until it all comes together. Scrape down sides if needed. Beat on medium speed for 2 minutes to aerate the batter.
Pour batter into prepared cake pans and bake for 22-32 minutes, or until cake tester comes out clean and top springs back to your touch.
Cool in pan on wire rack for 10 minutes then remove from pans and cool completely.
For the frosting:
Melt the chopped chocolate and butter in a good-sized bowl either in the microwave or suspended over a pan of simmering water. Cool.
Place powdered sugar into the food processor and blitz to remove lumps. Add remaining frosting ingredients and process until smooth.
Spoon about 1/3 of the frosting onto the center of the cake-half and spread with a knife or spatula until you cover the top of it evenly, leaving about 1/2" from the edges. Set the other cake on top, right side up, pressing gently to sandwich the 2 together.
Spoon another 1/3 of the frosting onto the top of the cake. Spread the sides of the cake smooth then spread the frosting on top in a swirly, textured way.
Hint: For my swirled top, I hold the icing spatula over the top outside edge and turn the plate as I go, working my way into the center.