An egg-shape stencil; piping bags; an open coupler or medium round tip.
Trace twelve 2-by-2.5-inch egg shapes onto a half sheet of parchment in 3 rows of 4. Repeat with another sheet of parchment. Line 2 baking sheets with the parchment, pencil-side down, and set aside.
Place the confectioners' sugar and almond meal in a food processor. Pulse until very finely ground, about 30 seconds. Sift the mixture into a medium bowl, discarding or saving for another use any large chunks of almond that to remain behind (don’t push them through). Set aside.
Place the egg whites, vinegar and salt in the clean bowl of an electric mixer and whip on medium speed until they look foamy, about 1 minute. Very slowly sprinkle in the granulated sugar. Raise the speed to medium-high and whip until the whites are shiny and form soft peaks. Add the vanilla and a large drop of food coloring. Continue whipping the whites until they form stiff peaks with tips that stand straight up. The meringue will gather in a mass around the whisk attachment when it is ready.
Transfer the meringue to a large mixing bowl. Fold in the almond-and-sugar mixture in 3 additions with a rubber spatula. Spread the mixture up the sides of the bowl then scrape down to the middle. You want the batter to be the consistency of soft-serve ice cream and run off the spatula in a thick stream like lava and hold a figure 8 in the bowl. Don't over mix.
Spoon the batter into a piping bag fitted with an open coupler or medium open round tip. Pipe
the batter onto the prepared sheet pans using the stencils as a guide. After all the eggs are piped, rap the tray on the counter 4 or 5 times to help remove any air bubbles. Tap the underside of the pan with your hand to pop remaining bubbles that you see on the surface of the macarons. Use a toothpick to pop any large bubbles. Let the macarons rest, uncovered, until they have lost their shine and aren't sticky when you gently touch the edge of one, 30 to 45 minutes.
Preheat the oven to 325 degrees F. Bake the macarons in the center of the oven, one sheet pan at a time, until the tops no longer move when nudged gently, 16 to 18 minutes, but making sure that the macarons don't brown. If the tops slide around, bake them for another 2 to 3 minutes.
Cool the macarons completely on the parchment. Fill one piping bag with 2 cups of the buttercream and another piping bag with the prepared jam. Pipe a ring of buttercream 1/4 inch from the edge on the flat bottom of a macaron, then fill the hole with the jam. Press another macaron on top to sandwich together. Repeat with the remaining macarons.
Divide the remaining 1/2 cup buttercream between two small bowls. Tint the buttercream to your desired color with gel food coloring, then transfer to piping bags fitted with small round piping tips, such as a number 2 or 3. Decorate with the extra buttercream as desired. Store in an airtight container in the refrigerator for up to 3 days.