Preheat the oven to 350 degrees F. Line muffin pans with cupcake liners.
Combine all dry ingredients (in the cupcake ingredients list) in a large bowl and whisk to combine.
Add in the butter and mix on low speed until the mixture resembles coarse crumbs.
Stir in the eggs, Greek yogurt, and vanilla, scraping down the sides of the bowl to incorporate everything.
Fill 16 line muffin cups halfway (about ¼ cup batter). A small ice cream scoop works great for this.
Bake for 15 minutes, or until a toothpick inserted in the center of a cupcake comes out clean. Let the cupcakes cool completely before frosting.
For The Frosting
Beat butter for 2 minutes with a mixer with the paddle attachment on medium speed.
Add 3 cups of powdered sugar and turn your mixer on the lowest speed (so the sugar doesn't blow everywhere) until the sugar has been incorporated with the butter.
Increase mixer speed to medium and add vanilla extract, salt, and 2 tablespoons of milk/cream and beat for 3 minutes. Add peanut butter and mix for 1 minute to ensure that it's incorporated thoroughly. If your frosting needs a more stiff consistency, add remaining sugar. If your frosting needs to be thinned out, add remaining milk 1 tablespoons at a time.