Preheat oven to 375°F. Butter 2 (8- by 2-inch) round cake pans and line bottoms of each with rounds of wax or parchment paper. Butter paper and dust pans with flour, knocking out excess. Sift together flour, baking soda, and salt. Stir together milk and lemon juice (mixture will curdle).
Beat butter in a large bowl with an electric mixer until creamy. Gradually add sugar, beating until pale and fluffy. Add eggs 1 at a time, beating well after each addition. Alternately add flour mixture and milk mixture in batches, beginning and ending with flour, mixing at low speed until just combined.
Divide batter between pans, smoothing tops. Bake in middle of oven until a tester comes out clean, about 20 minutes. Cool in pans on racks 10 minutes, then invert onto racks, remove paper, and cool completely.
+4 oz cream cheese, softened
+4 oz butter, softened
+3 1/2 cups confectioners sugar
+2 tablespoons fresh lemon juice
+1 or 2 teaspoons lemon zest
Beat everything together until fluffy and frost onto the cake layers!