Preheat oven to 375 degrees F. Line a cookie sheet with parchment paper; set aside.

In a large bowl, beat butter with an electric mixer on medium to high speed for 30 seconds. Add sugar, baking soda, and salt. Beat until combined, scraping bowl occasionally. Beat in egg and vanilla until combined. Alternately add flour and buttermilk, beating on low speed after each addition just until mixture is combined. Spoon dough with a teaspoon, 1 inch apart onto prepared cookie sheet.

Bake in the preheated oven for 7 to 8 minutes or until tops are set. Cool completely on parchment-lined cookie sheet on a wire rack. Peel cooled cookies off the paper. Spread the flat side of half of the cookies with about 1/4 teaspoon raspberry preserves. In a large pastry bag fitted with a small star tip, pipe White Chocolate and Mascarpone Filling over the raspberry preserve layer. Top with the remaining cookies, flat sides down. If desired, pipe additional filling on top of whoopie pies and/or sprinkle with colored sugar. Chill for 30 minutes before serving. Makes about 72 sandwich cookies.

In a heavy small saucepan, combine 3 ounces chopped white baking chocolate (with cocoa butter) and 1/4 cup whipping cream. Cook and stir over low heat until chocolate nearly melts. Remove from heat; stir until smooth. Cool for 15 minutes. Meanwhile, in a large bowl, combine 1/2 cup mascarpone cheese and 1/4 cup softened butter. Beat with an electric mixer on medium to high speed until smooth. Beat in 1/2 teaspoon vanilla. Gradually add 4 cups powdered sugar, beating well. Beat in the cooled white chocolate mixture. Chill about 30 minutes or until firm enough to pipe.

Place bars in a single layer in an airtight container; cover. Store in refrigerator for up to 3 days or freeze for up to 3 months. Thaw at least 1 hour in refrigerator before serving.