Stir together all ingredients except chocolate, pound cake, and butter until smooth, then stir in chocolate and chill 30 minutes.
Sandwich filling (2 heaping tablespoons each) between slices of cake to form 6 sandwiches. Smooth sides if necessary. Freeze on a parchment-lined baking sheet, covered, until very firm, at least 1 hour.
Melt 1/2 tablespoon butter in a 12-inch nonstick skillet over medium heat, then cook sandwiches, turning once and adding remaining 1/2 tablespoon butter, until golden, about 5 minutes total.
