Preheat oven to 350°F.

Grease or spray a 9” round springform pan. Line bottom of pan with parchment paper. (Note: The pan needs to be at least 3” deep.)

Using a medium bowl, whisk together cocoa powder and boiling water until smooth; set aside.

Using a separate large bowl, add flour, sugar, baking powder, baking soda and salt; whisk until well combined.

Add egg, egg yolk, vegetable oil, sour cream and vanilla; whisk until well combined.

Finally, add the cocoa powder and hot water mixture to the bowl in several additions, whisking well between each addition.

Pour batter into prepared baking pan; tap pan lightly on countertop to release any air bubbles.

Bake at 350°F for 20-25 minutes, or until a toothpick inserted into center of cake comes out clean.

Let cake cool in pan until completely cool. (Note: Do not release springform pan.)

Using a small bowl, whisk together cocoa powder and hot water; set bowl aside.

Using a double boiler or a heatproof bowl over a gently simmering saucepan of water, add chocolate. Heat, stirring occasionally, until fully melted. Remove chocolate from heat and stir in the cocoa powder mixture. Set aside and let cool for 5 minutes.

Meanwhile, whip the heavy cream and powdered sugar together until stiff peaks form.

Gently stir ⅓ of the whipped cream into the chocolate mixture. Using a spatula, fold in the remaining whipped cream.

Using an offset spatula, spread mixture evenly into the pan (on top of cake layer). Tap pan firmly on counter several times to remove any air bubbles. Cover pan and place in refrigerator for 30 minutes.

Using a small bowl, sprinkle gelatin over cold water; let sit for 5 minutes.

Place the white chocolate chips in a medium heatproof bowl; set bowl aside.

Using a microwave-safe measuring cup, heat ½ cup of whipping cream until boiling. Stir the gelatin mixture into the hot cream until fully dissolved. Pour hot cream over the white chocolate chips and let stand for 5 minutes.

Stir the white chocolate mixture until smooth; set aside to cool.

Meanwhile, whip remaining 1¼ cups of whipping cream together with powdered sugar until stiff peaks form.

Gently fold ⅓ of the cooled chocolate mixture into the whipped cream. Repeat process two more times with the remaining chocolate.

Using an offset spatula, spread mixture evenly into the pan (on top of chocolate layer). Tap pan firmly on counter several times to remove any air bubbles. Cover and refrigerate for at least 4 hours, or overnight.

Before releasing springform pan, slide a thin knife or spatula between the cake and the pan. Remove pan and smooth sides of cake with offset spatula if needed.

Using a medium saucepan (or a large microwave-safe measuring cup), heat whipping cream until boiling.

Place semi-sweet chocolate chips in a medium heatproof bowl. Pour hot whipping cream into bowl and stir gently until chocolate chips have completely melted and mixture is smooth.

Let chocolate ganache cool until pourable but not entirely liquid. (Note: I let mine cool for ~5 minutes.) Pour ⅓ of the ganache over the top of the cake. Use an offset spatula to smooth top.

Let chocolate ganache cool another ~5 minutes, or until it’s thickened enough to spread on sides of the cake. Use an offset spatula to spread ~½ of the remaining ganache on sides. Place cake back in refrigerator.

Let remaining ganache cool for 20-25 minutes, or until stiff enough to pipe. Transfer ganache into a piping bag fitted with a star tip. Pipe stars around the outside top edge of the cake. (I had extra ganache, so I went ahead and piped stars all over the top of the cake. Feel free to get creative here!)

Refrigerate cake for 2 hours before slicing. (Tip: To slice, run a knife under hot water for several seconds. Make the slice and then run knife under water again. Wipe knife clean after each cut.)

Store any leftover cake in the refrigerator.