Add the spice cake mix, eggs and pumpkin purée to a large mixing bowl. Beat with an electric hand mixer until evenly combined. Use a wooden spoon or spatula to fold in the butterscotch chips.

Divide the batter evenly among the prepared muffin cups, filling each approximately two-thirds full.

Bake standard muffins for 16–20 minutes. Jumbo muffins may require additional baking time. The muffins are ready when the tops spring back when lightly pressed or a toothpick inserted into the center comes out clean or with a few moist crumbs.

Allow the muffins to cool in the pan for approximately 5 minutes, then transfer them to a wire rack to cool completely.

To make the vanilla glaze, whisk the powdered sugar, milk and vanilla extract together until smooth. Drizzle the glaze over the cooled muffins and allow it to set before serving.