Make the Caramel Bun Glaze

In a small saucepan over high heat, stirring constantly to prevent scorching (or in a 1 cup microwavable measure with a spout, lightly coated with nonstick cooking spray), boil the raisin-soaking syrup until it is reduced to 1½ to 2 tablespoons/22.5 to 30 ml. In the microwave it will take about 3 minutes, but watch carefully toward the end to prevent scorching. Add the butter and stir until melted.

Glaze and Coat the Buns with Caramel

Remove the pans from the oven and set them on wire racks. Brush the buns evenly with the glaze. Unfold the aluminum foil ends from the outsides of the pans and use the ends to lift the buns out of the pans. Use a small pancake turner to slip under each bun to dislodge it from the foil and lift it, carefully separating it from the adjoining buns. Set each bun slightly apart from the other. The buns can be coated with caramel while they are still hot or at room temperature.

Have ready a 2 cup microwavable measure with a spout, lightly coated with nonstick cooking spray.

In a heavy 6 cup saucepan, preferably nonstick, stir together the sugar, corn syrup, and water until all of the sugar is moistened. Heat, stirring constantly, until the sugar dissolves and the syrup is bubbling. Stop stirring completely and let it boil undisturbed until it turns a deep amber (360°F/180°C, or a few degrees below because the temperature will continue to rise).

Remove it from the heat, and as soon as it reaches temperature, slowly and carefully pour the hot cream into the caramel. It will bubble up furiously. Use a silicone spatula or wooden spoon to stir the mixture gently, scraping the thicker part that settles on the bottom. Return it to a very low heat, continuing to stir gently for 1 minute, until the mixture is uniform in color and the caramel fully dissolved.

Remove it from the heat and gently stir in the butter until incorporated. The mixture will be a little streaky but will become uniform once cooled and stirred.

Pour the caramel into the prepared glass measure and let it cool for 3 minutes. Gently stir in the vanilla and let it cool to room temperature, stirring it gently once or twice.