3⁄4 cupmolasses (do not use blackstrap molasses here as it is too bitter)
1⁄2 teaspoonbaking soda
1 1⁄2 cupssugar (10 1/2 ounces)
3⁄4 cupvegetable oil
3 largeeggs, beaten
2 tablespoonsfresh ginger, finely grated (a rasp style grater makes quick work of grating the ginger)
1⁄4 cupcrystallized ginger, chopped (optional)
FOR THE FROSTING: Whisk sugar, flour, cornstarch, and salt together in medium saucepan. Slowly whisk in milk until smooth. Cook over medium heat, whisking constantly and scraping corners of saucepan, until mixture is boiling and is very thick, 5 to 7 minutes. Transfer milk mixture to wide bowl and let cool completely, about 2 hours. (Note: Transferring the milk mixture to a wide bowl will ensure that it cools within 2 hours.).
FOR THE CAKE: Adjust oven rack to middle position and heat oven to 350 degrees. Grease and flour two 8-inch round cake pans and line pans with parchment paper; set aside). Whisk flour, cocoa, ground ginger, baking powder, cinnamon, salt, pepper, and cayenne together in large bowl; set aside. Whisk coffee, molasses, and baking soda in second large bowl until combined. To the coffee/molasses mixture add sugar, oil, eggs, and fresh ginger to coffee mixture and whisk until smooth.
Whisk coffee mixture above into flour mixture until smooth. Pour 1 1/3 cups batter into each prepared pan. (Note: Baking four thin cake layers two at a time eliminates the need to halve thicker layers.). Bake until toothpick inserted in center of cake comes out clean, 12 to 14 minutes. Let cakes cool in pans on wire rack for 10 minutes. Invert cakes onto wire rack and peel off parchment; reinvert cakes. Wipe pans clean with paper towels. Grease and flour pans and line with fresh parchment. Repeat baking and cooling process with remaining batter.
TO COMPLETE THE FROSTING: Using stand mixer fitted with paddle, beat butter on medium-high speed until light and fluffy, about 5 minutes. Add cooled milk mixture and vanilla; mix on medium speed until combined, scraping down bowl if necessary. Increase speed to medium-high and beat until frosting is light and fluffy, 3 to 5 minutes.
TO FINISH: Place 1 cake layer on platter or cardboard round. Using offset spatula, spread 3/4 cup frosting evenly over top, right to edge of cake. Repeat stacking and frosting with 2 more cake layers and 1 1/2 cups frosting. Place final cake layer on top and spread remaining frosting evenly over top and sides of cake. Garnish top of cake with crystallized ginger, if using. Refrigerate cake until frosting is set, about 30 minutes. (Cake can be refrigerated, covered, for up to 2 days. Let cake come to room temperature before serving.).