1Preheat oven to 350 degrees F (175 degrees C). Grease 2 (9-inch) round cake pans.
2Press 2 cups of salmonberries through a metal mesh strainer into a bowl using the back of a large spoon; discard contents of strainer. Pour salmonberry pulp and juice (it will be about 1 1/4 cups) from bowl into a 2-cup measuring cup; pour in enough water to bring the pulp and juice to 2 cups. Stir well.
3Place cake mix, eggs, vegetable oil, and 1 cup salmonberry mixture in a bowl; blend with an electric mixer on low speed until moistened, about 30 seconds. Beat cake mixture at medium speed for 2 minutes. Fold remaining 1 1/2 cups whole salmonberries into cake mixture; pour evenly into prepared pans.
4Bake in the preheated oven until a toothpick inserted near the center comes out clean, about 30 minutes.
5While cake is baking, cook remaining 1 cup salmonberry mixture, remaining 1/2 cup whole salmonberries, and sugar in a saucepan over medium high heat, stirring constantly, until mixture begins to simmer; remove from heat.
6To assemble: While cake is still warm, gently poke holes all over the top of the cake layers with a fork. Pour half the cooled salmonberry mixture over each cake layer; allow to cool completely. Allow to cool completely.
Salmonberries grow wild along the west coast of North America from northern California to Alaska. Raspberries are from the same family and would make a good substitute if you cannot find salmonberries.
For cupcakes: line 24 muffin cups with paper liners; fill 2/3 full with cake mixture. Bake in the preheated oven until a toothpick inserted near the center comes out clean, about 19 minutes. While still warm, poke holes into top of each cupcake with a fork; spoon salmonberry mixture over each cupcake.
For a sweeter, drier cake omit the topping. The topping is very tart. You can add whole salmonberries for embellishment.