You can use a coffee scoop, which equals about 2 Tbsp., to measure batter into paper baking cups.
Beat butter at medium speed with an electric mixer until creamy. Gradually add granulated sugar, beating until light and fluffy. Add eggs, 1 at a time, beating after each addition. Add peanut butter, beating until smooth.
Combine flour, baking soda, and salt; add to peanut butter mixture alternately with buttermilk, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Stir in vanilla extract.
Spoon 2 Tbsp. batter into each of 24 paper baking cups in muffin pans. Dollop 1 rounded tsp. of your favorite flavor jam in center of batter in each cup. Top evenly with remaining batter (about 2 Tbsp. in each cup), covering jam.
Bake at 375° for 18 to 20 minutes or until golden brown. Let cool in pans on wire racks 5 minutes. Remove from pans, and cool on wire racks. Dust with confectioners sugar. Serve warm or at room temperature.