Preheat the oven to 350°F and grease a 9x13 pan.
In a large bowl or the bowl of a standing mixer, whisk together the flour, sugar, baking soda, and salt. In a small bowl, whisk the eggs, buttermilk and vanilla together, set aside.
Bring the water and 1 ½ sticks unsalted butter to a simmer in a medium sized sauce pan. Stir in ½ cup peanut butter and oil. Stir until the peanut butter is completely incorporated then remove from the heat and add to the dry ingredients on low speed. Increase the mixer speed to medium as the wet ingredients mix.
Slowly add the remaining wet ingredients to the batter and beat on medium-high speed for 1 minute, scraping down the sides of the bowl half way through. Pour the batter into the prepared 9x13 pan and bake for 33 to 36 minutes or until a toothpick inserted in the center of the cake comes out clean.
Let the cake cool for 15 minutes, then evenly spaced poke holes ¾ of the way through the cake. I did 4 holes across the width of the cake and 6 holes down the length. (See notes for tools ideas to poke the holes.)
Spread the remaining ½ cup peanut butter over the cake while the cake is warm but not hot. You need a little bit of warmth for the peanut butter to melt into the holes but you do not want it to melt into the cake. Let the cake cool completely before frosting with chocolate buttercream - this can be done in the refrigerator to speed up the process.