1. Preheat oven to 400°F. Prepare two standard muffin tin with 18 paper liners.
  2. In a large bowl, beat peanut butter and unsalted butter with an electric mixer until creamy. Add the brown sugar and beat until well combined. Add the eggs and vanilla, and continue beating until light and creamy.
  3. In a separate bowl, whisk flour, baking powder, baking soda, sea salt, and ground cinnamon. Add one third of the dry ingredients to the creamed mixture and beat on low speed just until combined. Add half of the milk, and beat again. Add another third of the remaining dry ingredients, followed by the rest of the milk, and ending with the remaining dry ingredients. Continue mixing just until batter comes together. Do not over-mix.
  4. Using a small ice cream scoop or spoon, fill the wells of the muffin tins about 1/4 full. Use the tip of a spoon to push the batter to cover the entire bottom and a little up the sides. Place one rounded teaspoon of jam in each of the 18 wells, making sure to keep it in the center as much as possible. Top the jam with the remaining batter, making sure it covers the jam.
  5. Bake for 15-20 minutes, or until the tops are golden brown and spring back when gently pressed. Let cool in pan for 10 minutes, then remove and serve.