In a large glass measuring cup, mix together the vinegar, milk, and sour cream. Set aside.
Mix dry ingredients (baking soda through flour) together in a small mixing bowl and set aside.
In a large mixing bowl, cream together butter and sugar until light and fluffy. Add eggs, 1 at a time, mixing well after each addition. Mix in vanilla extract.
Add the flour mixture alternately with the mlk mixture, beginning and ending with flour mixture. Stir in ½ cup blackberry jam.
Scoop about ⅓ cup of batter into 12 paper lined cupcake cups. Distribute evenly any remaining batter (there won't be much if any).
Bake for 20-23 minutes, until cake tester inserted into center of cupcake comes out clean. Cool completely.
To make frosting cream together butter and shortening. Add 3 cups of powdered sugar and blend well. Blend in seedless blackberry jam. Blend in vanilla extract and more powdered sugar if needed. Frost cooled cupcakes with buttercream.