Transfer quartered lemon to a food processor, and process until finely chopped, about 1 minute. Measure out 1/3 cup lemon pulp, then discard any excess pulp. Return 1/3 cup pulp to processor.

Combine flour, baking powder, and salt; add to butter mixture alternately with ¾ cup milk, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Stir in 1 ...