Preheat oven to 350°. Cream butter and peanut butter together, gradually adding sugar and molasses until fluffy. Then add eggs one at a time, beating well after each addition. Combine flour, baking powder, and salt; add to creamed mixture alternately with milk. Begin and end with flour mixture. Mix well after each add-in, then stir in vanilla extract.
Pour cake batter into two 8" parchment lined cake pans and shake a bit to get rid of any air bubbles and level. Bake for 26 to 30 minutes- cake tester should emerge with a few moist crumbs and cake should be firm with some spring. Let cool in pan for 10 minutes then transfer to cooling rack.
Frosting
In a large bowl or stand mixer, beat butter until very fluffy. Add peanut butter, salt, and vanilla extract and beat until fully incorporated. Slowly add powdered sugar while continuing to beat mixture. Add milk until desired consistency is reached
*Assembly. Cut the top (mound) off of one cake layer. Make sure your cut is as level as possible. Spread half of frosting on top, then place the second layer atop frosting. Using an icing spatula (or knife) dipped in warm water spread remaining icing around cake for a "naked cake" look. Only spread in one direction, occasionally dipping your tool in warm water to avoid crumbs. Garnish with roasted peanuts if desired.
Notes
Feel free to use a lighter molasses if you'd prefer a sweeter taste.
If you prefer a full coverage, traditional frosting application, simply double the peanut butter frosting recipe.