Preheat the oven to 175 degrees C. Grease and flour two round cake pans; line bottoms with parchment paper.
Whisk flour, sugar, cocoa, baking powder, baking soda, and salt together in a large bowl. Add eggs, milk, oil, and vanilla; beat until combined. Pour cake batter into the prepared pans.
Bake until a toothpick inserted in the centres comes out clean, about 35 minutes. Cool layers in pans on wire racks for 10 minutes. Run a table knife around the edges to loosen and invert carefully onto racks to cool completely, 1 to 2 hours.
While cake layers bake, drain cherries for topping, reserving 1/2 cup juice. Combine reserved juice, cherries, sugar, and cornstarch in a 2-quart saucepan. Cook over low heat until thickened, stirring constantly. Stir in vanilla. Let cool at room temperature for 30 minutes. Transfer to the refrigerator to cool completely before assembling the cake.
Combine whipping cream and confectioner’s sugar for frosting in a chilled medium bowl. Beat with an electric mixer at high speed until stiff peaks form.
Split each cake layer in half horizontally using a long serrated knife. Tear one split layer into crumbs; set aside. Gently brush loose crumbs off the top and sides of the remaining layers using a pastry brush or your hands. Reserve 1 1/2 cups frosting for piping decorations on the cake; set aside.
To assemble, place one cake layer on a cake plate. Spread with 1 cup frosting; top with 3/4 cup cherry topping. Top with a second cake layer; repeat layers of frosting and cherry topping. Top with a third cake layer. Frost sides of cake.
Pat reserved cake crumbs onto frosting on the sides of the cake. Spoon the reserved 1 1/2 cups of frosting into a pastry bag fitted with a star decorator tip. Pipe around the top and bottom edges of the cake. Spoon the remaining cherry topping on top of the cake.
Store covered in the refrigerator until ready to serve.