Preheat oven to 350°. Line 16 muffin cups with paper or foil liners.
In a large bowl, beat butter, marzipan and sugars until blended. Add eggs, one at a time, beating well after each addition. Beat in vanilla. In another bowl, whisk flour and baking powder; gradually beat into butter mixture. Fold in crushed pineapple.
Fill prepared cups two-thirds full. Bake 20-25 minutes or until a toothpick inserted in center comes out clean. Cool in pans 10 minutes before removing to wire racks to cool completely.
In a small heavy saucepan, melt chocolate and butter over very low heat; stir until smooth. Cool slightly until mixture thickens to a spreading consistency. Spread over cupcakes. Top with pineapple.