Preheat oven to 325. Prepare a large tube or bundt pan by greasing and flouring it. Set pan aside.
Only fill your bundt or tube pan up about 3/4 full. It will run over if you put all the batter into the pan. Don't let the extra batter go to waste though make some muffins or a small loaf.
In a large bowl mix flour, baking powder, salt and baking soda. Set aside.
In a medium bowl cream together butter and sugar until fluffy. Add the brown sugar and peanut butter and continue mixing.
Add the eggs one at a time. Wait until one egg is fully mixed in before adding the next one. Add vanilla.
Add the dry ingredients and the milk to the creamed mixture. Alternate between the flour mixture and the milk using about a 1/4 of the ingredient at a time. Always begin and end with the dry ingredients. Mix completely.
Pour batter into the prepared pan and bake for 60-70 minutes. Start checking to see if is done at 60 minutes using a wooden toothpick.
Continue checking every 5 minutes to make sure it does not overcook. When the toothpick comes out clean the cake is done.
Remove from the oven and let sit for 15 minutes. Then remove it from the cake pan and cool completely on a wire rack. Dust with powdered sugar or frost as directed.
Note that this recipe makes a large cake. For smaller pound cakes use loaf pans or even mini bundt cake molds. Adjust the cooking time accordingly since they will not need as much time to bake.
For the frosting cream butter and peanut butter. Add powdered sugar and milk while mixer is set on low. Add the milk 1 tablespoon at a time until you are happy with the consistency. Pour over the top of the cake.
For chocolate peanut butter frosting melt 4 ounces of semisweet chocolate squares. Cool slightly and add to the butter and peanut butter mixture. Proceed with the rest of the instructions.
Don't worry if the frosting is to thick or thin just add small amounts of milk to the thin or powdered sugar to thicken.