Heat oven to 375F.Line the bottoms of two 8 or 9-inch cake pans with parchment paper and lightly butter.Sift the flour, cocoa, baking powder, baking soda, and salt together.Stir the buttermilk, pumpkin, and vanilla extract together in a small bowl.Beat the butter and sugars together using an electric mixer on medium speed until light and fluffy.Beat in the eggs and yolk, one at a time.Reduce the mixer speed to low and alternately beat in the flour and buttermilk mixtures in thirds.
Pour the batter into the prepared pans.
Bake until a tester, inserted into the center, comes out clean--about 35 mins (NOTE: if you're using nonstick cake pans, check around 25 mins as they cook faster).Cool layers completely before icing.Frosting:.Beat cream cheese using a mixer on medium-high speed until fluffy.
Add 1 1/2 cups confectioners' sugar, cocoa, cinnamon and vanilla and beat on slow speed until well combined.In a separate bowl, beat heavy cream with 3/4 cup confectioners' sugar and the food coloring on medium-high speed to soft peaks.Gently fold the whipped cream into the cream cheese mixture until well combined.
Spread 1 cup frosting between the two layers and use the remaining frosting to ice the top and sides of the cake.Chill for 30 mins and proceed to the glaze.Glazing:.
Place chopped bittersweet chocolate, butter, and corn syrup in a medium heatproof bowl. Bring heavy cream to a boil, pour it over the chocolate, and let sit for 3 minutes.Gently stir, using a whisk, until smooth.
Let sit for 3-5 min, until the glaze thickens slightly.
Pour the glaze onto the center of the frosted cake and use the back of a spoon to spread it out to the edges of the cake, allowing some to spill over.
