Preheat oven to 300°. Coat only the bottom of a 9" tube pan with a bit of butter and flour (sides should remain ungreased so the cake will adhere to them better when rising). Put butter in a large mixing bowl and work it with a wooden spoon until it becomes shiny, about 5 minutes. Add sugar and salt and continue to work sugar and butter together until well mixed, then stir in a circular motion until the mixture loses most of its gritty feeling. Adding eggs will dissolve the rest. Add eggs one at a time, stirring well after each addition. After third egg has been incorporated, add 2 tbsp. flour. Add the last two eggs and continue to stir, then add the rest of the flour in four parts, stirring well after each addition. Beat in vanilla and lemon juice.
Spoon batter into prepared tube pan and smooth top. Bake for 40 minutes, then increase temperature to 325°. After 5 more minutes of baking, gently remove pan from oven, bend your ear to it, and listen for bubbling or a slight sound. If you hear something, put it back in oven to bake another 10 minutes, then remove again and listen. Let it bake another five minutes if necessary, until cake is quiet. Or insert a skewer into cake; if it comes out clean, the cake is done.
Remove cake from oven and run a knife around the sides of the pan; turn out onto a wire rack, then turn cake face up. Cool uncovered for 15 minutes, then cover with a clean towel (otherwise the cake will become dry and hard). When cool, store in a clean cake tin.