Combine the butter and brown sugar in a saucepan over medium heat. With a rubber spatula or wooden spoon, stir constantly as the butter melts. Once butter has melted, switch to a whisk to make sure the two are really combined. (Otherwise, you might see an oily layer around the edges where they’re still a bit separated; you do not want that.)

Once melted butter and brown sugar are combined, add 3 Tablespoons (45ml) of heavy cream and the salt. Whisk to combine, then stop whisking and attach a candy thermometer to your pan, making sure the bulb is not touching the bottom of the pan (as you’ll get an inaccurate reading).

Let the caramel boil, giving it a quick whisk every 30 seconds or so, until the temperature reaches 230°F (110°C). On my stove, it takes just under 2 minutes.

Remove the saucepan from heat, give it another quick whisk, and let cool for a minute. Carefully transfer the salted caramel to a heat-proof mixing bowl set on a cooling rack and cool for just 20 minutes before continuing. The caramel sauce thickens during this time.

Sift in confectioners’ sugar and add remaining 2 Tablespoons (30ml) of heavy cream. With a handheld or stand mixer fitted with a whisk attachment, beat on medium-low speed until the ingredients are incorporated. Turn mixer up to high speed and beat for 1 minute. If frosting is very thick, beat in 1 more Tablespoon of heavy cream. Avoid over-mixing or the frosting will begin to “break” and separate.

Use it right away as a thin glaze or icing, or let it sit and thicken. It’s going to crust a bit as it sits, which is fine—simply give it a stir when you’re ready to use it. If you want to pipe this frosting, let it sit for at least 20–30 minutes before filling your piping bag, to give it a chance to thicken up to a pipeable consistency.

Frost cake or cupcakes and garnish with a drizzle of salted caramel sauce, if desired.