Preheat the oven to 350°F. Line cupcake pans with 16 paper liners.

In a large mixing bowl, whisk together the melted coconut oil, brown sugar, vanilla, eggs, and pumpkin puree until smooth and evenly combined.

Add the flour, pumpkin pie spice, baking powder, salt, and baking soda.

Gently whisk just until the dry ingredients are incorporated and the batter is smooth. Do not overmix.

Divide the batter evenly between the prepared cupcake liners, filling each approximately two thirds full.

Bake for 18 to 20 minutes, or until the tops are set and a toothpick inserted into the center comes out clean or with a few moist crumbs.

Allow the cupcakes to cool in the pans for about 5 minutes. Transfer them to a wire rack and cool completely before frosting.

Add the softened butter to the bowl of a stand mixer or a large mixing bowl. Beat until light and creamy.

Add the powdered sugar, vanilla, cinnamon, and heavy cream.

Beat on low speed until combined, then increase to medium high speed and beat until the buttercream is light, smooth, and fluffy.

Add additional heavy cream, 1 teaspoon at a time, when needed to reach a soft, pipeable consistency.

Spread or pipe the cinnamon buttercream over the completely cooled cupcakes.