Combine butter and sugar; whisk in buttermilk, puree, and eggs: In another bowl, whisk together butter and sugar until smooth, then whisk in buttermilk, pumpkin purée, eggs, and vanilla to thoroughly combine.

Mix wet mixture into dry; add cranberries: Stir wet mixture into dry mixture just until no dry flour remains. Fold in cranberries.

Divide batter between muffin cups; top with pepitas: Spoon batter into muffin cups (about 1/2 cup each) and sprinkle pepitas over the top of each.

Bake: Bake until a toothpick inserted into the center of a muffin comes out clean, 22 to 25 minutes. Let cool in tin on a wire rack 10 minutes, then turn out muffins onto rack. Serve warm.