Preheat oven to 350°F and 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.
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.
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.