For the Crumb Topping

1. In a medium bowl, whisk together the light brown sugar, granulated sugar, ground cinnamon, salt, and melted butter until well combined.
2. Continue whisking for about one to two minutes, allowing the sugars to start dissolving slightly.
3. Switch to a wooden spoon and stir in the all-purpose flour. The mixture will become crumbly and somewhat paste-like. Set this aside while you prepare the muffin batter.

For the Muffins

1. Preheat your oven to 350°F (175°C). Line a standard 12-cup muffin tin with paper liners. Lightly spray the inside of the liners with nonstick cooking spray. This helps prevent the dense batter from sticking. (Alternatively, you can skip liners if you thoroughly grease the muffin cups, but ensure muffins cool completely before removing).
2. Combine the 6 tablespoons (86g) unsalted butter and the peanut butter in a microwave-safe bowl. Heat on high for 30 seconds, then stir thoroughly. If not fully melted and smooth, microwave in additional 15-second bursts, stirring after each, until ready. Set aside to cool slightly.
3. In a separate medium bowl, whisk together the 1 3/4 cups (212g) all-purpose flour, baking soda, 1/4 teaspoon (1.5g) salt, and 1/4 teaspoon (0.6g) ground cinnamon. Set this dry mixture aside.
4. In a large bowl, whisk the large egg, 1/2 cup (110g) packed light brown sugar, and vanilla extract until smooth.
5. Pour the slightly cooled peanut butter and butter mixture into the egg mixture. Stir until everything is smooth and well combined. The batter will be somewhat thick at this stage.
6. Add the dry ingredients to the wet ingredients. Stir until they are almost combined (the batter will be very thick).
7. Pour in the milk and continue stirring just until a cohesive batter forms. Be careful not to overmix.
8. Divide the batter evenly among the prepared muffin liners, filling each about three-quarters full. An ice cream scoop works well for uniform portions.
9. Spoon about a tablespoon of strawberry preserves into the center of the batter in each muffin cup. Gently press the preserves down into the middle. It’s perfectly fine if some goes towards the sides.
10. Generously sprinkle the prepared crumb topping over the batter in each cup.
11. Bake for 18 to 22 minutes, or until a wooden skewer inserted into the muffin part (avoiding the jam) comes out clean or with moist crumbs attached.
12. Let the muffins cool in the pan for a few minutes before transferring them to a wire rack. If you baked without liners, allow them to cool completely in the pan before gently lifting them out with an offset spatula or spoon. If using liners, try to let them cool significantly before peeling the liner away to prevent sticking.