Chocolate and cherries are a match made in sweets heaven. A touch of kirsch -- brandy made from cherries -- enhances the filling for a more pronounced cherry flavor.
Whisk together 1/3 cup sugar, kirsch, and cornstarch in a medium pot. Add cherries and any juices. Bring to a boil and cook, stirring constantly, over medium-high until thickened, about 4 minutes. Remove from heat, and let cool to room temperature.
Preheat oven to 350 degrees. Butter a 9-by-13-inch metal baking pan. Line with parchment paper, leaving a 2-inch overhang on the long sides of the pan. Butter parchment.
Whisk cocoa with hot water in a medium bowl until smooth. Whisk in sour cream and melted chocolate; let cool. Whisk together flour, baking soda, and salt in another medium bowl.
In the bowl of an electric mixer fitted with the paddle attachment, beat butter and 1 2/3 cups sugar on medium speed until light and fluffy, 3 to 4 minutes. Add eggs, one at a time, beating to combine after each and scraping down sides of bowl as needed. Beat in vanilla. With mixer on low, add flour mixture in two parts, alternating with the cocoa mixture and beginning and ending with the flour; beat until combined. Fold in 1 1/4 cups cherry filling; reserve remaining filling for topping.
Pour batter into prepared pan; smooth with an offset spatula. Bake until a cake tester inserted in center comes out clean, about 50 minutes. Transfer pan to a wire rack to cool completely. Remove cake from pan.
Transfer cake to a serving platter. In a small saucepan over low, heat gelatin and 2 tablespoons water over low, stirring constantly, just until gelatin melts, about 3 minutes. In the bowl of an electric mixer fitted with the whisk attachment, whisk heavy cream and remaining 1/4 cup sugar on medium-high. Once soft peaks form, slowly add gelatin mixture with the mixer running until combined. Frost top of cooled cake and top with remaining cherry filling.