Preheat oven to 350 degrees. Butter a 14-by-3-inch round cake pan or two 10-by-3-inch round cake pans. Line with parchment cut to fit, and butter parchment. Whisk together flour, baking soda, and salt.
Simmer stout, molasses, and cherries in a saucepan, stirring occasionally, until cherries are plump, about 5 minutes. Strain, reserving liquid. Let cherries cool completely. Return liquid to pan, and bring to a boil. Remove from heat and whisk in 11/2 cups cocoa until smooth. Transfer to a bowl, and let cool slightly.
Beat eggs and sugars with a mixer on medium-high speed until combined, 2 to 3 minutes. Reduce speed to low. Gradually beat in stout mixture. Raise speed to medium, and beat until combined. Reduce speed to low again, and beat in oil and vanilla. Add flour mixture in three additions, alternating with the sour cream, and beat until combined.
Toss cherries with remaining 1/4 cup cocoa, and gently fold into batter. Pour batter into prepared pan. Gently tap pan on counter to eliminate some of the air bubbles (batter will still look bubbly).
Bake cake until a toothpick inserted into center comes out clean, 60 to 65 minutes for 14-inch pan and 50 to 55 minutes for 10-inch pans. Let cool in pan on a wire rack for 20 minutes. Run a knife around edges of pan to loosen, and invert cake onto rack to cool completely. Carefully remove parchment, and turn cake right side up. Using a serrated knife, trim rounded top of cake to create a flat surface. Transfer cake, cut side down, to a wire rack set over a baking sheet.
Make the glaze: Heat sugar, cornstarch, and the water in a large saucepan over medium heat, whisking until sugar has dissolved. Add chocolate, and bring mixture to a simmer, whisking constantly and scraping down sides of pan as needed. Cook until smooth and thick, about 8 minutes. Remove from heat. Whisk in more water, a teaspoon at a time, until glaze is thick but pourable.
Using a ladle, spoon glaze onto center of cake (2 cups glaze for 14-inch cake and 1 cup glaze for each 10-inch cake). Spread to the edge with an offset spatula, but do not allow glaze to drip down sides. Let stand, uncovered, at room temperature overnight to allow glaze to harden.
Place a stencil on top of cake, and sift cocoa over top. Carefully remove stencil.
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