Special equipment: Stand mixer and 4 (3/8-pound) mini loaf pans
Preheat oven to 350 degrees F.
Butter and flour mini loaf pans. In a small bowl, whisk together flour, baking powder, baking soda and salt. Mix together the sour cream and espresso until dissolved.
In a stand mixer fitted with the paddle attachment, cream butter and sugar on medium speed until light and fluffy. Add eggs and vanilla and mix until well incorporated. On low speed, add half the flour mixture. Mix until just combined. Add half the sour cream and mix until combined. Repeat with remaining flour and sour cream. Pour into prepared loaf pans filling halfway with batter. Bake 40 minutes or until a toothpick comes out clean when inserted in the center. Cool in pans 10 minutes. Remove from pans and cool on racks.
For the frosting: In stand mixer, mix together butter and peanut butter over medium high speed until smooth. Add confectioners' sugar and mix until smooth.
Top mini loaves with peanut butter frosting.