Preheat oven to 350 degrees F (175 degrees C). Lightly grease an 8×4 inch baking pan.
Sift together flour, soda and salt. In a large bowl, cream sugar and peanut butter together. Beat in egg and vanilla until smooth. Stir in flour and milk alternately, beating until smooth after each addition. Spoon batter into prepared pan.
Bake in preheated oven for 1 hour, until well browned. Remove from the pan to cool. Store in a covered container.