Preheat oven to 350 degrees and set a rack in the middle level. Butter and flour a 9 by 13 by 2-inch pan.
Beat the butter by machine with the sugar. Beat in eggs one at a time. Beat in the extract. Mix the flour with the salt and baking soda and add it alternately to the batter with the sour cream, beginning and ending with the flour. Scrape down bowl and beater often during this stage of mixing.
Scrape the batter into the prepared pan and bake the cake about 40 minutes, or until it is well risen and deep golden and a toothpick inserted in the center emerges clean.
Cool the cake in the pan on a rack for a few minutes, then unmold and cool completely. Wrap cake tightly in plastic wrap for storage.
For the whipped cream, combine all ingredients and whip on medium speed until stiff.
To finish the cake, place the layer on a cardboard or platter and spread with the cream. Arrange the blueberries in the upper left corner of the cake, as on the starred field of a flag, then use the raspberries or strawberries to make the red stripes, leaving stripes of uncovered whipped cream between the stripes of berries. Chill cake until just before serving.