Preheat oven to 350°F and spray two 8-inch round cake pans with nonstick spray.
Whisk together flour, cocoa, baking powder, baking soda, and salt in a large bowl; make a well in the center of the mixture. Add C&H® Granulated Sugar, C&H® Light Brown Sugar, eggs, egg yolk, milk, yogurt, oil, and vanilla to flour mixture; whisk in just until incorporated. Add boiling water and chopped chocolate. Whisk until smooth.
Divide batter evenly between cake pans and smooth with an offset spatula. Tap pans sharply to reduce air bubbles.
Bake for 28 to 30 minutes, until cakes spring back to the touch, a toothpick inserted in the center comes out with a few crumbs, and the edges are pulling slightly away from the pan. Cool cakes completely in pans on a wire rack. Once completely cool, transfer cake layers to the refrigerator and chill for 30 minutes.
Beat butter and salt in a large mixer bowl on medium speed until smooth. Beat in vanilla and peppermint extracts. Gradually mix in C&H® Powdered Sugar on low speed until incorporated, scraping down bowl as needed. Add heavy cream and mix in on low speed until smooth. Increase speed to medium-high and beat frosting for 1 minute, until light and airy.
Place chopped chocolate and salt in a medium heat-proof bowl. Bring heavy cream to a simmer in a small saucepan, heating until bubbles form at the edge of the pan. Pour warm heavy cream over chocolate. Cover the bowl and let stand for 1 minute, then stir with a spatula until chocolate is completely melted and mixture is smooth.
Place one cake layer on a rotating cake stand and use an offset spatula to swirl frosting over the cake. Pipe a ring of frosting around the outer edge of the cake layer and chill the layer for 10 minutes.
Fill the center of the cake layer with 3 tablespoons chocolate ganache, smoothing with an offset spatula. Sprinkle 2 tablespoons crushed peppermint candy over the ganache. Spread a thin layer of frosting on the second cake layer, and place the layer (frosting side down) over the ganache and crushed peppermint candy. Chill the stacked cake for 20 minutes.
Use an offset spatula to apply a layer of frosting to the top and sides of the cake while you rotate the cake stand, then use a bench scraper to gently smooth the edges of the cake and remove any excess frosting. Chill the frosted cake for 20 minutes.
Re-warm remaining ganache in the microwave for 10-15 seconds, stirring until spreadable. Top cake with ganache, smoothing with an offset spatula. Sprinkle 2 tablespoons crushed peppermint candy over the top of the ganache. If desired, pipe any remaining frosting over the top of the cake and sprinkle with crushed peppermint candy.
If desired, gently press crushed peppermint candy into buttercream around the sides of the cake.