Have a pastry bag fitted with ½-inch round tip ready.
Whisk together the powdered sugar, almond meal and salt until they are uniform in color. Sift the mixture 3 times, discarding any bits that are too large to pass through the sifter, and set aside.
180 g powdered sugar, 140 g almond meal, 1 pinch fine salt
In a small bowl whisk together the baker’s sugar and dehydrated egg whites, set aside.
60 g baker’s sugar, 5 g dehydrated egg white powder
In a large bowl, beat egg whites until foamy, continue beating, while gradually adding the sugar and dehydrated egg white mixture until all of the sugar mixture has been added.
100 g large egg whites
Add the lemon juice and raspberry baker's emulsion and continue beating until the mixture is shaving cream consistency.
10 ml lemon juice, 4 drops raspberry bakers emulsion
Add the food coloring, starting with 2 drops and adding one drop at a time until it is the desired shade of blue.
2-4 drops sky blue food coloring
Carefully fold the dry ingredients into the egg whites in 3 batches. Continue to fold until the batter falls from your spatula in a continuous ribbon and slowly sinks back into itself.
Scrape the batter into the pastry bag and pipe 1” circles onto the prepared baking sheets. Rap the sheets on the counter firmly a few times, then let sit for about 20 minutes until a skin forms on the surface and they are no longer tacky to the touch.
Bake 15 – 20 minutes, cool completely before removing from baking sheets.
For the macaron filling:
In a medium bowl, beat the butter, heavy cream, lemon juice and salt until creamy and lightened in color.
112 g unsalted butter, 8 ml heavy cream, 24 ml lemon juice, 1/8 tsp fine salt
On low speed, beat in the powdered sugar a little at a time until it has all been incorporated. Increase the mixer speed to high and continue beating until the frosting is a light, fluffy, spreadable consistency.
240 g powdered sugar
Add food coloring, 1 drop at a time, until the frosting reaches the desired shade of blue (it should match the macaron shells, but keep in mind that the frosting will get darker as it dries/sets)
1-2 drops sky blue food coloring
Scrape the blue lemon buttercream into a piping bag and put the raspberry jam in another piping bag.
200 g raspberry jam
To assemble:
Pipe a thin dam of blue lemon buttercream around the outer edge of the bottoms of half of the macaron shells. Then pipe a small amount of the raspberry jam into the center of each, inside the buttercream dam. Sandwich with the remaining half of the macaron shells.
Notes
If you expect these macarons may be served more than a day or two after assembly, you may want to spread a thin layer of the buttercream on the surface of the macaron shells before piping the buttercream "dam." This will create a barrier between the raspberry jam and the macaron shell preventing the shell from becoming soggy.