If you have a piping bag, find the 1/2-inch diameter tip. If you don't have one, you can create one with a plastic bag or parchment paper. Either way - make sure it's ready before you begin.

Line one or two baking sheets with parchment paper.

Combine the powdered sugar and the almond flour in a large bowl and sift together.

In another bowl, beat the egg whites until soft peaks form.

Beat in the sugar and yellow food coloring (if you are using). The peaks should be stiff and glossy.

Add the almond flour mixture in two batches, mixing in a circular motion just enough till the batter forms a thick ribbon running off a spoon or spatula. Be careful not to over mix.

Pour the batter into the pastry bag, and pipe out in rounds about 1 1/4-inch around, apart 1-inch apart on the baking sheet.

You'll want to release air bubbles from the batter, so bang the baking sheet on the counter a couple of times. You can also pop any air bubbles you see with a toothpick.

Let the macarons sit at room temperature for about half an hour. You want a kind of skin to develop on them.

Preheat the oven to 325 °F.

Bake the macarons one tray at a time on the middle rack for about 15 to 18 minutes, rotating the pan about halfway through.

Place a foil tent over the macarons if they start to brown.

Remove from the oven, and let them cool about 15 minutes before adding the filling.

Beat the softened butter until it is light and lemon colored.

Divide the powdered sugar into three or four parts, and add each one at a time, beating until thoroughly incorporated.

Add the sour cream and salt as you continue to beat.

Add the lemon juice and finely grated lemon zest and beat for another minute or two, until the buttercream is fluffy in texture.

Add it to the clean piping bag.

Flip over half the cookies on the baking sheet(s), and pipe each with a generous amount of the buttercream.

Sandwich the cookies together, and enjoy.