Pick over and rinse 1 lb dried navy beans (2 1/4 cups). Soak in cold water to cover by 2 inches at least 8 hours. Alternatively, quick-soak beans (see cooks' note, below). Drain.
Put oven rack in middle position and preheat oven to 350°F. Rinse and pat dry 1 (1/4-lb) piece salt pork (rind discarded), then cut into 3 pieces.
Put beans and pork in an ovenproof 3-quart heavy pot with a lid. Add 4 1/2 cups water, 1 large onion (finely chopped; about 2 cups), 2 chopped garlic cloves, 1/2 cup Grade B maple syrup, 1 tablespoon dry English mustard (preferably Colman's), and 1 teaspoon freshly ground black pepper and stir to combine. Cover pot and bake until beans are just tender, 3 to 4 hours.
Reduce oven temperature to 325°F. Remove lid. Bake beans, stirring occasionally, until most of liquid is absorbed but beans are still saucy, 1 to 1 1/2 hours more. Remove from oven. Stir in 1 tablespoon cider vinegar and 1 teaspoon salt or more to taste.
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.
Make frosting:
Beat cream and confectioners sugar with cleaned beaters until it just holds stiff peaks. Fold one third of whipped cream into lemon curd to lighten, then fold in remaining whipped cream.
