1 cup unsalted butter, cold and cut into small pieces
1/2 cup plus 2 Tablespoons buttermilk
For the filling:
4 ounces Mascarpone or cream cheese, room temperature
1 egg yolk
2 Tablespoons granulated sugar
1 egg white plus 1 teaspoon water, whisked together
Preheat oven to 375° F. Line a rimmed baking sheet with parchment paper or grease.
For the crust, combine flour, sugar and salt in a food processor fitted with a steel blade. Add butter and pulse until it is the size of small peas. With machine running add buttermilk and pulse until dough forms a ball. Do not over mix. Wrap in plastic and chill for 30 minutes.
For the filling combine Mascarpone (or cream cheese), egg yolk and sugar and mix until uniform.
Roll dough 1/8-inch thick. Cut into 24 (3-inch) rounds. Place 12 rounds on prepared sheet.
Top with 1 teaspoon each of jam and filling. Brush edges with egg white mixture. Cut small cross in center of remaining pastry rounds. Place on top and crimp edges with a fork. Chill 10 minutes.
Brush tops with egg white mixture and sprinkle with sugar. Bake 20-25 minutes or until golden brown. Serve warm or at room temperature.