This is a delicious and sturdy cake, perfect for transporting to a picnic.
Beat butter and sugar at medium speed with an electric mixer until fluffy. Add eggs, 1 at a time, beating until blended after each addition. Stir in vanilla.
Combine flour and next 4 ingredients; add to butter mixture alternately with buttermilk, beginning and ending with flour mixture. Beat at low speed until blended after each addition. Fold in jam. (Do not overmix.) Pour batter into a greased 10-inch Bundt pan.
Bake at 350° for 1 hour or until a wooden pick inserted in center comes out clean. Cool in pan on wire rack 20 minutes; remove from pan, and continue cooling. Serve with blackberries and Créme Fraîche.