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1
Preheat oven to 350 degrees and set a rack in the middle level.
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2
Butter and flour a 9 by 13 by 2-inch pan.
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3
Beat the butter by machine with the sugar.
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4
Beat in eggs one at a time.
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5
Beat in the extract.
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6
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.
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7
Scrape down bowl and beater often during this stage of mixing.
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8
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.
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9
Cool the cake in the pan on a rack for a few minutes, then unmold and cool completely.
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10
Wrap cake tightly in plastic wrap for storage.
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11
For the whipped cream, combine all ingredients and whip on medium speed until stiff.
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12
To finish the cake, place the layer on a cardboard or platter and spread with the cream.
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13
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.
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14
Chill cake until just before serving.
