Place white chocolate morsels and butter in a medium-sized heat-safe bowl.
Heat heavy cream in the microwave on 100 percent power (high) for 1–2 minutes, until the heavy cream begins to steam and bubble but doesn’t boil over. Immediately, pour the heavy cream over the chocolate. Make sure all of the chocolate is submerged under the cream. Allow the mixture to sit, without stirring, for 3 minutes.
Add lemon peel, lemon juice, vanilla extract, poppy seeds, and fine sea salt to the chocolate, and whisk until the mixture is smooth and fully blended. If there are still bits of chocolate or butter after stirring with the cream for 60–90 seconds, heat the entire mixture in the microwave on 50 percent power (medium) for an additional 15–30 seconds, then stir until the correct consistency forms.
Transfer ganache to a shallow container, and press plastic wrap against the surface of the ganache (to prevent a skin from forming on top). Place the ganache in the fridge, and allow the mixture to cool until it’s firm enough to pipe, about 1–2 hours.
Match up macaron shells so they are paired with a similar size/shaped shell. Pipe filling on one side of the macaron shell using a ½ inch (1.3 cm) round piping tip. Gently place the matching macaron on top of the filling.
Mature the macarons with the filling in the refrigerator overnight. This will allow the macarons to slightly soften and develop a deeper flavor.