Preheat oven to 325°. Line 48 mini-muffin cups with paper liners. In a large bowl, cream butter and sugar until light and fluffy. Add eggs, 1 at a time, beating well after each addition. Beat in orange zest, orange juice and vanilla. In another bowl, whisk flour, baking powder and salt; add to creamed mixture alternately with buttermilk, beating well after each addition.
Fill prepared cups two-thirds full. Bake until a toothpick inserted in center comes out clean, 11-13 minutes. Cool in pans 5 minutes before removing to wire racks to cool completely.
For buttercream, in a large bowl, beat butter and salt until creamy. Gradually beat in confectioners' sugar, milk and vanilla until smooth.
Using a paring knife, cut a 1-in.-wide cone-shaped piece from top of each cupcake; discard removed portion. Fill cavity with marmalade. Pipe or spread buttercream over tops.