Red Velvet Sandwich Cookies

Name: Red Velvet Sandwich Cookies
Ingredients: 4 oz. unsweetened chocolate, chopped; 2 c. granulated sugar; 2 sticks salted butter, at room temperature; 2 large eggs; 3 tbsp. buttermilk; 5 tsp. red food coloring; 1 tbsp. vanilla extract; 2 1/2 c. all-purpose flour; 1 tsp. baking powder; 1/2 tsp. kosher salt; 4 oz. white chocolate, chopped; 8 oz. cream cheese, at room temperature; 2 c. powdered sugar; 2 tbsp. unsalted butter, at room temperature; 1 tsp. vanilla extract; White nonpareils, for decorating