Preheat the oven to 325°F. Line two baking sheets with parchment paper.: As the oven warms you will notice a faint electrical hum and gradually increasing warmth in the kitchen, which primes the baking environment for even heat. Preheating is important because it gives the macaroons the immediate hot environment they need to set the exterior quickly while retaining chew inside. A common mistake is not waiting for full preheat, which can lead to uneven color and longer bake times, so only bake when the oven has reached temperature.
In the bowl of a standing mixer, whisk egg whites, sugar, vanilla, and salt on high speed until thick and opaque white (about 1 minute).: The sound will change from thin whisking to a fuller, higher pitched beat as air is incorporated, and you will visually see the mixture go from glossy liquid to a thick, opaque foam. This texture traps air that lifts the macaroons, so do not shortchange the whipping. Overwhipping until grainy is a mistake to avoid; stop when peaks are stable but still smooth.
Use a spatula to fold the coconut and flour into the egg white mixture.: Folding should be slow and deliberate, with the spatula scraping the sides and bringing the mixture up through the center, preserving the trapped air. You will feel the mixture thicken as the coconut absorbs moisture, and it should come together into a cohesive, slightly sticky mass. Rushing this step or stirring vigorously will deflate the foam and yield denser cookies, so be patient and gentle.
Use a 1 1/2 tablespoon cookie scoop or a rounded spoon to drop compact balls onto prepared baking sheets.: When scooping, the mass should cling together and hold its shape without sagging, indicating the right moisture balance. The sound of the spoon scraping against the bowl is minimal as the mixture is cohesive. If you leave the mounds too loose they will spread and lose height, so pack them slightly when forming.
Bake for approximately 25 minutes, rotating the pans during baking, until cookies are golden brown.: During baking the kitchen will fill with toasty coconut aromas and the tops will shift from pale to a warm golden color, particularly at the edges. Rotating the pans ensures even browning since oven hotspots can make one tray darker. Pull them when the tops are lightly golden, because overbaking dries them out and makes them crumbly instead of chewy.
After 1 to 2 minutes, transfer cookies to a cooling rack and cool completely.: The cookies continue to set as they cool, and you may hear a subtle settling sound as steam escapes. Cooling on a rack prevents moisture from accumulating underneath, which keeps the bottoms from going soggy. A common error is leaving them on the hot sheet too long, which can lead to overbrowning and loss of chew.
In the top of a double boiler, melt chocolate chips and shortening until smooth and fully melted.: As the chocolate melts you will see it transform from a solid mound to a silky, glossy pool, with a warm chocolate scent rising. The gentle steam heat prevents scorching, and the shortening helps the texture become shiny and fluid for dipping. Avoid direct high heat which can seize the chocolate into a grainy mess, a signal that it was overheated.
Transfer melted chocolate to a shallow bowl if needed to make it easier to dip. Dip the bottom of each cooled macaroon into the chocolate, just enough to coat the bottom and up the sides a bit. Allow the excess chocolate to drip off the bottom. Place a dipped cookie on a sheet of parchment paper on a baking sheet.: When dipping you will notice the chocolate cling to the ridged coconut surface, creating a glossy coat that contrasts the matte texture of the top. Let gravity help the excess drip away so the coating is even and not globby. If the chocolate is too thick, it will pool; if too thin, it may not cover adequately, so aim for a viscous ribbon consistency.
Allow the chocolate to set (place the baking sheet in the fridge for a few minutes to help it set more quickly).: As the chocolate cools it will firm up with a slight snap, and the fridge can speed this process so you can serve sooner. Be careful not to leave the cookies in the fridge too long, as condensation can form when they return to room temperature, which can dull the chocolate shine. A typical mistake is rushing to stack them while the chocolate is not fully set, which can mar the finish.