Separate the egg whites from the yolks and let the whites sit at room temperature until no longer chilled.
Double sift the almond flour and confectioners’ sugar together into a large bowl and set aside.
Wipe the bowl and whisk of a stand mixer with a paper towel dampened with white vinegar, then dry well.
Place the egg whites and cream of tartar in the clean bowl and whip on medium low speed until foamy, about 3 minutes.
With the mixer running, slowly add the granulated sugar in 3 additions, beating about 45 seconds between each addition.
Increase the mixer speed to medium high and whip until stiff, glossy peaks form, about 10 to 11 minutes total; add the light blue gel food coloring when the meringue reaches soft peaks and continue whipping.
Sift the almond flour and confectioners’ sugar mixture over the meringue and gently fold with a silicone spatula until the batter flows in thick ribbons that disappear back into the bowl in about 10 to 15 seconds.
Transfer the macaron batter to a piping bag fitted with a 1/2 inch round tip.
Line light colored sheet pans with parchment paper and place macaron templates underneath if using. Pipe 1.5 inch circles, holding the bag straight up and twisting your wrist to stop the flow to avoid peaks.
Firmly tap each pan on the counter several times to release air bubbles, then use a toothpick to pop any remaining visible bubbles.
Let the piped shells rest at room temperature until a dry, matte skin forms and the tops do not stick to your fingertip, about 60 minutes or longer if humid.
Preheat the oven to 300°F. Bake one sheet at a time on the center rack for 19 to 23 minutes, covering loosely with foil if the shells begin to brown after the feet form.
Check doneness by gently nudging a shell; it should not wiggle and the tops should feel set, not sticky. Cool the shells completely on the pans.
In a small bowl, stir the cool water and brown gel food coloring together. Lightly flick the mixture over the cooled shells with a clean food-safe brush to create a speckled look, then let dry completely.
Place the chocolate morsels and butter in a heat safe bowl. Heat the heavy cream in a small saucepan over medium heat until steaming with small bubbles at the edges, then pour over the chocolate and butter.
Let the mixture sit undisturbed for 3 minutes, then whisk until smooth and glossy.
Whisk in the finely pulsed Cadbury mini eggs, vanilla bean paste, and fine sea salt until fully combined and thick.
Transfer the ganache to a shallow container, press plastic wrap directly onto the surface, and refrigerate until thickened and pipeable, about 30 to 60 minutes.
Gently remove the macaron shells from the parchment and pair them by similar size.
Transfer the ganache to a piping bag fitted with a small round tip and pipe a dollop onto the flat side of half of the shells.
Top each filled shell with a matching shell and twist gently to spread the ganache to the edges.
Place the assembled macarons in an airtight container and refrigerate overnight to mature and develop flavor.
Bring the macarons to room temperature for about 20 to 30 minutes before serving.
Store leftovers in an airtight container in the refrigerator for up to 7 days.