Preheat oven to 350°F. Line an 8x8-inch pan with parchment paper or foil and grease lightly.

Melt butter in a large microwaveable bowl in the microwave. Add peanut butter and brown sugar and whisk to combine. Add the egg, vanilla, and whisk to combine. Add the flour and stir until just combined, taking care not to over-mix. Fold in the strawberries. Pour batter into prepared pan, smoothing it lightly with a spatula or offset knife. Dollop the strawberry preserves over the top in tablespoon drops and swirl into the batter.

Bake for 25 minutes, or until a toothpick inserted into the center comes out clean. Allow bars to cool for at least 2 hours before slicing and serving into 16 squares. I like to refrigerate them before cutting to make them easier to cut. Bars can be stored in an airtight container at room temperature or in the refrigerator for up to 1 week, or in the freezer for up to 3 months