To make the caramel sauce: In a deep, heavy-bottomed saucepan that holds at least 2 quarts, gently stir together the sugar, water, salt, and cream of tartar. From this point on, swirl the pan instead of stirring; this will help prevent crystals from forming.

Set the saucepan over medium-high heat. When the mixture comes to a boil, reduce the heat to medium and monitor the color of the caramel closely (no walking away!). The syrup will begin to turn golden at the edges; to even out the color, swirl the pan gently without lifting it off the heat.

Cook the mixture until it’s a uniform light golden brown, 5 to 8 minutes.

Remove the caramel from the heat, immediately add the butter, and stir with a flexible spatula. After the butter has melted, heat the cream briefly in the microwave for 15 seconds, until warm. Slowly pour in the cream and continue stirring. Add the vanilla and stir until the sauce is smooth.

Pour the caramel sauce into a small bowl or storage container, cover, and chill in the refrigerator, at least 4 hours and up to 2 weeks.

To finish the whipped caramel frosting: In a medium bowl or the bowl of stand mixer fitted with the whisk attachment, place 1 cup (about 285g) of the caramel sauce. Allow it to warm up at room temperature for 10 minutes to ensure it whips easily.

Add the butter to the bowl and mix at low speed to combine, about 1 minute. Increase to high speed and whip for 2 to 3 minutes, until light and fluffy.

If the frosting feels dense, curdled, or greasy, it’s too cold. Gently warm the mixture by setting the bowl over a pan of steaming water for a few minutes (just until you see the edges begin to soften/melt), then re-whip until smooth.

If the frosting feels soupy or loose, it’s too warm. Chill for 15 to 20 minutes in the refrigerator before re-whipping.

Use the Whipped Caramel Frosting as desired to decorate the single-layer cake or cupcakes of your choice. You’ll have about ½ cup leftover caramel sauce; serve it alongside slices of your cake or spoon over ice cream.

Storage information: Store frosting in an airtight container at room temperature for up to 5 days; freeze for longer storage. Store caramel sauce in an airtight container in the refrigerator for up to 2 weeks.