Preheat oven to 350ºF. Line muffin pan with cupcake liners.
In a medium bowl, stir together canned pumpkin, oil, granulated sugar, milk, and vanilla extract. Sift in the all-purpose flour, baking soda, baking powder, salt, and spices. Gently whisk by hand until just combined.
Once combined, fold in the mini chocolate chips.
Fill liners with 1/4 cup batter (cups should be approximately 2/3 full). Bake for 22 to 24 minutes, or until toothpick inserted into the center comes out clean.
Transfer to a wire rack and let cool completely before frosting.
For frosting, in a heatproof bowl set over (not in) a pan of simmering water, whisk together egg whites, light-brown sugar, and salt. Cook, whisking constantly, until the sugar has dissolved and the mixture is warm to the touch.
Transfer to the clean bowl of an electric mixer. Beat on medium speed until fluffy and cooled, about 15 minutes.
Raise speed to high; beat until stiff peaks form. Reduce speed to medium-low; add unsalted butter, 2 to 3 tablespoons at a time, until fully incorporated.
Spread or pipe onto cooled cupcakes.