Mix butter, sugar, cocoa, flour, and egg until the dough just comes together.
Chill the dough for 30 minutes, then press into a tart pan.
Bake at 350°F (175°C) for 18–22 minutes until firm. Cool completely.
Melt chocolate for the croustillant layer and gently mix with praline and crisp flakes.
Press the croustillant lightly into the cooled tart shell. Chill for 10–15 minutes.
Warm the cream until just hot, not boiling.
Pour cream over chopped chocolate and let sit for 1 minute.
Stir gently until smooth, then blend in butter carefully without adding air.
Pour ganache into the tart shell and level gently.
Chill for 2–3 hours until softly set.
Warm glaze if using and pour smoothly over the tart.
Chill 30–45 minutes, decorate lightly, then slice and serve.