Preheat oven to 450°F.
In a medium bowl, whisk together the flour, baking powder, granulated sugar, and salt.
In a separate small bowl, combine the buttermilk, brown sugar, and cinnamon. Stir until the sugar is mostly dissolved.
Pour the wet mixture into the dry ingredients. Stir gently until just combined—be careful not to overmix.
Fold in ¾ cup of raisins, reserving ¼ cup for the topping.
Pour the melted butter into an 8×8-inch baking dish. Carefully spoon or spread the biscuit dough over the butter, smoothing it out gently to reach the edges.
Sprinkle the reserved raisins on top.
Score the dough into 9 squares (a 3x3 pattern) using a butter knife. This will help guide your cuts after baking.
Bake for 28–30 minutes, or until the top is golden brown and a toothpick inserted in the center comes out clean.
While the biscuits are baking, make the glaze. In a small bowl, whisk together the confectioners’ sugar and milk until smooth. If the glaze is too thick, add a little more milk—½ teaspoon at a time—until it reaches a pourable consistency.
Once the biscuits are out of the oven, top the warm biscuits with the glaze.
Let sit for 5 minutes before cutting and serving.