Preheat the oven to 350 degrees. Grease and lightly flour a 10-inch tube pan. Cut out a piece of waxed paper to fit in the bottom and grease and flour the paper. Stir together the flour, salt and baking soda. In a mixing bowl, beat the butter until light and creamy. Gradually add the sugar, beating until light. Add the eggs, one at a time, beating for a minute after each addition; scrape the bowl often. Add the lemon extract and vanilla; beat well. Add the dry ingredients and sour cream alternately, starting and ending with the flour, beating after each addition until just combined. Pour the batter into the prepared pan. Bake about 1 to 1 1/2 hours or until a toothpick inserted in the middle of the cake comes out clean. Cool 15 minutes on a rack. Remove from the pan and cool. Peel off the paper.
Source: "New Southern Cooking", Nathalie Dupree, Alfred A. Knopf, Inc., 1986