Place a rack in the center of your oven and preheat the oven to 350 degrees F.
Coat an 8x8-inch baking pan with nonstick spray. Line with parchment paper, leaving some overhanging on two sides like handles, then coat with spray again.
In the bowl of an electric mixer fitted with the paddle attachment or a large mixing bowl, cream the butter, Greek yogurt, and sugar on medium speed until pale yellow and fluffy, about 1 to 2 minutes. Reduce the mixer speed to low, then add the vanilla, egg, and peanut butter and mix until all ingredients are well combined.
In a small bowl, stir together the white whole wheat flour, all-purpose flour, baking powder, and salt. With the mixer on low speed, slowly add the flour mixture to the peanut butter mixture. Mix just until the flour disappears.
Portion two-thirds of the dough into the prepared baking pan and spread over the bottom into an even layer with a knife or offset spatula. Spread the jam evenly over the dough. Break small pieces of the remaining dough evenly over the jam (you’ll have some bits of jam showing through). Sprinkle the chopped peanuts over the top. Bake for 35 to 40 minutes, until the bars are golden brown. Let cool 10 minutes in the pan, then, using the parchment paper handles, lift the bars onto a wire rack to cool completely. Slice and serve.
Store leftovers well wrapped at room temperature or in the refrigerator for about 5 days. Freeze for up to 3 months.
Vary it up: For a peanut-free version, try swapping the peanut butter and peanuts for almond butter and chopped toasted almonds. You can also use any flavor of jam you love, not just strawberry. I plan to try a blackberry or raspberry and almond version next!