In a medium heatproof bowl, place chopped chocolate. Set aside. In a small saucepan over medium heat, bring the heavy cream and butter to a low boil. Remove from the heat and pour mixture over the chocolate. Let stand for 1 minute. Using a rubber spatula, stir mixture until melted and smooth (if not completely melted, reheat in the microwave for several seconds and stir until melted). Pour ganache over chocolate tart crust and refrigerate until set, at least 1 hour.

Combine raspberries, sugar, lemon zest and juice in a saucepan over medium heat and bring to a boil, stirring constantly. This should take 5-10 minutes. Reduce heat and simmer 5 minutes. Remove from heat and strain mixture through a medium mesh strainer into a heatproof bowl. Stir constantly with a rubber scraper to ensure the mixture strains properly. Discard the seeds and remaining contents in the strainer.

In a small bowl or glass measuring cup, whisk together the egg yolks, heavy cream, and cornstarch until smooth. Stir yolk mixture into raspberry mixture, add a pinch of salt, and stir until well combined.

Using a double boiler or heat-proof bowl over a pot of simmering water (bain-marie), pour the raspberry and yolk mixture into the top pot of the double boiler or use the heat-proof bowl the mixture is already in.

Cook on moderate heat, whisking constantly, until mixture becomes thick (mine was ready in 10 minutes, but it can take up to 20). If you have a thermometer, it should register 170F/75C; otherwise, it should coat the back of a wooden spoon and leave a clear pass if you run your finger through it. The curd will thicken more once cooled.

Remove from heat and immediately strain mixture through a sieve. Add butter, a few cubes at a time, and whisk until completely melted and incorporated, and mixture is smooth. Take your time with it—the whisking makes for an airy and light texture. Allow to cool to room temperature before filling the tart.

Fill the tart with raspberry curd right over the ganache layer (if using) or right onto the crust then refrigerate for at least 4 hours until chilled.