1. Preheat oven to 350 degrees. Butter a 12-cup standard muffin tin. In a small bowl, using your fingertips, combine 2 tablespoons butter, brown sugar, and 1/2 cup flour; set aside.

2. In a large bowl, whisk together remaining flour, baking powder, baking soda, and salt. In a separate bowl, using an electric mixer, beat remaining 4 tablespoons butter and granulated sugar until light and fluffy. Add eggs one at a time; beat after each addition until combined. Add flour mixture and buttermilk in alternating batches, beginning and ending with flour mixture; beat until just combined. Fold in raisins.

3. Divide batter equally into tin; top with brown-sugar mixture. Bake until browned and a toothpick inserted in centers comes out clean, 20 to 25 minutes. Let stand 5 minutes before lifting muffins out onto a cooling rack. Serve warm or at room temperature.