Preheat oven to 425° F. Line a baking sheet with parchment paper.
In a small bowl, combine raisins with enough boiling water to cover. Let soak for 10 to 15 minutes before draining. Turn out well-drained raisins on a paper towel to absorb excess moisture. Set aside.
In the meantime, cut butter into ¼-inch pieces and return it to the refrigerator to keep it cold until needed.
Combine flour, sugar, baking powder, and salt in a large wide bowl. Mix to combine.
Add butter to the dry ingredients and toss it to coat with flour. Working quickly with clean fingers or a pastry blender, rub or cut butter into the flour mixture until it resembles coarse meal.
Add drained raisins to the mixture and toss to coat.
Drizzle the about 3/4 cups of the heavy cream evenly over the mixture. Toss with a fork, scraping up from the bottom until the dough starts to clump together. If needed, add remaining cream.
Knead the mixture together in the bowl until it comes together in a mass. It’s okay if there are a few stray bits and dry patches; the most important thing is to avoid overworking the dough.
Turn out the dough on a parchment-lined baking sheet. Press in stray bits gently and shape the dough into a 7 to 8-inch square about 3/4-inch thick. Don't worry if it looks a bit rough and ragged.
Using a knife or bench scraper, divide the square into 9 smaller squares. Divide each square in half diagonally to make 18 small triangles.
Separate the scones carefully to give them room to spread. Bake until pale golden, about 12 to 15 minutes. Let cool at least 10 minutes before glazing.
To make the glaze, mix confectioners' sugar, milk, and vanilla extract in a small bowl until smooth. Drizzle over the scones.