In a food processor, pulse to combine the flour, sugar, baking powder, salt, and cinnamon.
Add the cold, cubed butter, and process until only pea-sized bits remain visible.
Transfer the mixture to a bowl, and gently stir in the raisins and 1 cup buttermilk just until the dough begins to come together.
Transfer the dough to a clean surface and knead very lightly to combine.
Flour the surface.
Press or roll dough to a 10x5-inch rectangle.
Fold the dough like a letter, into thirds.
Press or roll the dough to 1-inch thickness, then cut out rounds using 2 ½-inch cutters.
Gently press the scraps together and cut out additional rounds.
Transfer the biscuits to a lined baking sheet and refrigerate while the oven preheats.
Preheat the oven to 400 F.
When the oven comes to temperature, brush the tops of the biscuits with buttermilk.
Bake for 18-20 minutes until the biscuits are golden brown and baked through.
In the meantime, to prepare the glaze, stir to combine the confectioners’ sugar, a pinch of salt, and 2 tablespoons buttermilk until thick but pourable. Add a splash of any remaining buttermilk if needed to adjust the consistency.
Let the biscuits cool at least 10 minutes, then top each with a spoonful of glaze.
Sprinkle with cinnamon if desired, and serve warm or at room temperature.