Make the Pastry Cream

1. Add the milk to a medium saucepan and heat over medium-low heat until just below a simmer. Remove from the heat, add the lavender, cover and let steep for 15 minutes.
2. Strain the milk and discard the lavender. Measure 1¼ cups (300 g) of the infused milk and return it to the saucepan. Heat over medium-low heat until steaming and just below a simmer.
3. Meanwhile, whisk together the sugar, cornstarch and salt in a medium bowl. Add the egg yolks and whisk until smooth.
4. Slowly ladle about half of the hot milk into the yolk mixture, whisking constantly. Gradually pour the tempered yolks back into the saucepan.
5. Cook over medium heat, whisking constantly, until the pastry cream thickens and comes to a gentle boil.
6. Remove from the heat and add the cold butter. Whisk or use an immersion blender to emulsify the butter into the pastry cream until completely smooth.
7. Strain the pastry cream into a bowl. Press plastic wrap directly against the surface and refrigerate until completely chilled, preferably overnight.

Make the Buttercream

1. Put the egg whites and sugar into the bowl of a stand mixer. Set the bowl over a saucepan of simmering water, making sure it does not touch the water. Heat, whisking constantly, until the sugar is dissolved and the egg whites reach 160°F. Fit the bowl onto the stand mixer with the whisk attachment and whisk on high speed until thick and glossy, 5 to 7 minutes.
2. Switch to the paddle attachment. Slowly add the butter, one tablespoon at a time, and continue beating until smooth and silky, about 10 minutes. If the buttercream looks soupy or curdled along the way, keep beating and it will come together. Add the honey, vanilla and flaky salt. Reduce the mixer to low and mix for 1 to 2 minutes to remove any air bubbles.