Preheat your oven to 350°F (175°C). Line a 24-cup muffin tin with paper liners, or grease and flour the cups.

In a large mixing bowl, cream together the packed brown sugar, vegetable oil, unsweetened applesauce, and creamy peanut butter until the mixture is light and fluffy.

Beat in the large eggs one at a time, then stir in the vanilla extract.

In a separate medium bowl, whisk together the all-purpose flour, baking soda, cream of tartar, and salt.

Gradually add the dry ingredients to the wet ingredients, alternating with the milk. Mix until just combined. Be careful not to overmix.

Spoon the batter into the prepared muffin cups, filling each about two-thirds to three-quarters full, as the cupcakes do not rise significantly.

Bake for 15 to 20 minutes, or until a wooden skewer inserted into the center of a cupcake comes out clean. The tops of the cupcakes should spring back when lightly pressed.

Allow the cupcakes to cool in the muffin tin for at least 10 minutes before carefully transferring them to a wire rack to cool completely. Frost once completely cooled.