To prepare Vanilla Bean Cake, preheat oven to 350 degrees.
Beat butter and shortening at medium speed with an electric mixer until creamy. Gradually add sugar, beating well. Add eggs, 1 at a time, beating until blended after each addition.
Combine flour, baking powder, and salt; add to butter mixture alternately with buttermilk, beginning and ending with flour mixture. Beat at low speed until blended after each addition. Stir in vanilla bean paste.
Place paper baking cups in 2 (12-cup) muffin pans, and coat with cooking spray; spoon batter into cups, filling about two-thirds full.
Bake for 12 to 15 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 10 minutes; remove from pans to wire racks, and cool completely. Makes 24 cupcakes.
To prepare frosting, beat cream at high speed with an electric mixer until stiff peaks form. Beat mascarpone, powdered sugar, vanilla, and salt in a large bowl at medium speed with mixer until blended. Gently fold whipped cream into mascarpone mixture until blended. Makes 3 cups. Frost each cupcake. Top each generously with white chocolate curls.
Chocolate curls make quite an impressive topping. At Dreamcakes Bakery, they recommend using a good quality milk chocolate or white chocolate bar without nuts or fillings. Soften it very slightly in the microwave about 10 seconds on HIGH; microwave again an extra 5 seconds at at time; if needed. Pull a sharp vegetable peeler along narrow end of bar, letting curls fall onto a chilled sheet pan. The secret is having the chocolate at just the right temperature and using gentle pressure with the vegetable peeler.