Heat the oven to 350°F (175°C). Grease and line two 9-inch round cake pans.
Whisk flour, cocoa, sugar, baking powder, baking soda, and salt in a large bowl.
Add eggs, milk, oil, and vanilla; beat until combined. Carefully mix in hot coffee. Divide the thin batter between pans.
Bake 28 to 32 minutes, until a tester in the center comes out with a few moist crumbs. Cool 10 minutes in pans, then turn out and cool completely.
For the filling, whisk 1 cup reserved cherry juice with 1/3 cup sugar and cornstarch in a saucepan. Cook over medium heat, stirring, until thick and glossy, 3 to 5 minutes. Fold in drained cherries and cool completely.
Beat cold cream with powdered sugar and vanilla to soft, spreadable peaks. Keep chilled.
Place one cake layer on a serving plate. Spread half the whipped cream over it and spoon on half the cherry filling, leaving a small border. Top with the second layer and repeat with remaining cream and cherries.
Garnish with chocolate shavings. Chill 30 minutes before slicing. Refrigerate leftovers promptly.