For macarons shells

1. Line two baking pans with parchment paper or silicone baking mats. 
2. If using whole or slivered almonds, grind them with the powdered sugar in a food processor until the mixture resembles sand. Stop to redistribute the mixture once or twice.
3. Place the egg whites and instant coffee paste (if using) in a large bowl, or the bowl of a stand mixer with a whisk attachment.
4. Combine granulated sugar and water in a small pot, and bring to a boil over medium-high heat.
5. While the sugar syrup is boiling, whip the egg whites to a foamy consistency. If the whites are foamy before the syrup is ready, turn the mixer on to the lowest speed to keep them moving while you wait for the syrup.  
6. When the syrup reaches 244-248° F (120° C), remove from heat and immediately drizzle it into the egg whites with beater running.
7. Turn beaters up to high, and whip egg whites until they cool to room temperature.
8. Add coffee and vanilla extracts, and whip a minute more.
9. Sift the dry ingredients directly into the meringue, folding them in gently but firmly. Scrape the sides and bottom of the bowl as you fold.
10. Fold until the batter ribbons off the spatula. Combined and thick, not over-mixed and runny. 
11. Transfer to a piping bag (or large plastic bag) fitted with a quarter-inch tip, pushing the bottom of the bag into the tip so it doesn’t leak. 
12. Pipe onto a silicone baking mat or parchment paper. 
13. Optional: Let the piped macarons rest on the counter for 10 to 30 minutes. The shell will turn from shiny and sticky, to smooth and dull. You'll be able to touch the top without damaging it.
14. Bake at 300° F for about 14 minutes, rotating the pan once halfway through baking. Macarons are done when you can't wiggle the top of the shells away from the pied (feet). 
15. Cool a few minutes before removing from baking mat or parchment.
16. Fill with buttercream and for maximum flavors and texture, store chilled for 24 hours before serving at room temperature.

For Coffee Buttercream Frosting

Beat butter on high speed until it lightens in color and increases slightly in volume. You can use a hand mixer, or stand mixer with a whip attachment. 
In a few additions, add confectioner's sugar, beating well to combine. 
Add extract and any instant coffee/espresso paste, and beat to combine thoroughly. 

To Assemble Macarons

Pick out your best-looking shells and reserve them for the tops.
Spoon buttercream into a pastry bag (small round, or no tip) or a small plastic baggie.  Cut a small hole in one corner, if using a baggie.
Pipe a small amount of buttercream into the center of half your macaron shells, filling the circle a little more than halfway. 
Gently press your tops on the bottoms. 
Serve immediately, or store refrigerated for up to a week. 
Also serve macarons at room temperature.