an 8-inch cardboard cake circle; a pastry bag fitted with a medium plain tip or a resealable plastic bag
For the cake: Preheat the oven to 350 degrees F.
Coat two 8-inch-round cake pans with cooking spray and line the bottoms with parchment paper. Whisk the cocoa powder and 1 1/2 cups boiling water in a medium bowl until smooth; set aside. Whisk the flour, sugar, baking powder, baking soda and salt in a large bowl until combined. Add the eggs, vegetable oil, sour cream and vanilla and beat with a mixer on medium speed until smooth, about 1 minute. Reduce the mixer speed to low; beat in the cocoa mixture in a steady stream until just combined, then finish mixing with a rubber spatula.
Divide the batter between the prepared pans and tap the pans against the counter to help the batter settle. Bake until a toothpick inserted into the middle comes out clean, about 35 minutes. Transfer to racks and let cool 10 minutes, then run a knife around the edge of the pans and turn the cakes out onto the racks to cool completely. Remove the parchment. Trim the tops of the cakes with a long serrated knife to make them level.
For the ganache: Put the chocolate in a medium bowl. Bring the heavy cream and corn syrup to a simmer in a small saucepan. Pour the hot cream over the chocolate and let stand 5 minutes, then stir until completely smooth. Let the ganache sit until it has cooled and thickened to a spreadable consistency, 20 to 30 minutes.
Place 1 cake layer on a cardboard cake circle on a wire rack set over a baking sheet to catch any drips. Spread about 1/2 cup of the ganache on top and place the second cake layer cut-side down over the ganache. Dollop half of the remaining ganache on top and spread with a large offset spatula so the ganache covers the top of the cake and slowly runs down the sides. Repeat with the remaining ganache, using the offset spatula to make a smooth, even layer over the surface and around the sides of the cake. Refrigerate until set, 30 minutes to 1 hour.
To decorate: Melt the white chocolate chips in a small microwave-safe bowl in the microwave in 15 second intervals on 50 percent power until melted, stirring after each interval. Let sit 5 minutes, then transfer to a pastry bag fitted with a medium plain tip. (Alternatively, transfer to a resealable plastic bag and snip off a corner.) Pipe a straight line through the center of the cake from top to bottom, then another line through the center from left to right, dividing the cake into quarters. Pipe 2 more lines to divide the cake into eighths. Pipe a circle of white chocolate around the edge of the cake. Pipe a small circle where the points intersect in the center of the cake. Place a candy-coated chocolate near the tip of each point for reindeer noses (one of them will be red). Add 2 reindeer eyes above each nose. Stick 2 pretzel twist pieces above each pair of eyes.
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