Prepare the pastry cream:
Mix together the egg yolks, vanilla, and sugar.
Add the cornstarch and dilute completely.
Heat the milk to a boil.
Pour the boiled milk over the egg yolk mixture and whisk vigorously to avoid lumps.
Return this mixture to the saucepan and bring to a boil again. Grate the tonka bean and mix. Set aside off the heat.
Prepare the caramel:
Pour the sugar into a double-bottomed saucepan; over low heat, caramelize until golden.
When the caramel is ready, pour it onto a silpat or parchment paper, making designs as desired for the eclair decoration. As it cools to room temperature, the caramel will set and harden.
Assemble the eclairs:
Split the eclairs in half. Cut the tops at an angle to form points. Set aside.
Fill a pastry bag with the tonka cream. Place the reserved tops.
Decorate with caramel designs and lemon balm leaves.
Sprinkle with powdered sugar.