Trim the long sides of each pound cake at an angle using a serrated knife. Use the knife to carve out a 1/2-inch-deep, 2 1/2-inch-wide channel down the center of each cake.
Cover the cakes with the frosting, leaving the bottom of the channel unfrosted. Use the back of a fork to make grooves in the frosting. Put the cakes in the freezer until the frosting sets, about 20 minutes.
Meanwhile, make the peanut butter filling: Beat the butter and peanut butter in a large bowl with a mixer on medium-high speed until light and fluffy. Beat in the confectioners' sugar on low speed until smooth. Set aside.
Mix a few drops of green food coloring in a small bowl with a drop of yellow food coloring and a splash of water. Brush onto the frosting. Adjust the color with more food coloring or water as needed.
Spread the peanut butter filling into the channel in each cake using the back of a spoon.
Arrange the prunes on top of the peanut butter filling.