Preheat your oven to 350°F (175°C) to ensure even baking.
Grease an 8x8-inch baking pan or line it with parchment paper.
In a mixing bowl, combine the softened butter, peanut butter, and brown sugar. Whisk until smooth and creamy, then add the egg and vanilla extract, whisking until well incorporated.
In a separate bowl, whisk together whole wheat pastry flour, baking powder, and salt. Gradually add this to the wet ingredients, alternating with the milk, mixing until just combined.
Fold in 1/3 cup of your favorite jelly or jam into the batter.
Pour the batter into the prepared baking pan and smooth the top. Bake for 25-30 minutes, or until a toothpick comes out clean.
Allow the cake to cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely. Slice and serve with powdered sugar or a dollop of extra jelly, if desired.