Make the shortcakes: Line a rimmed baking sheet with parchment paper. Pulse the flour, sugar, sesame seeds, tahini, baking powder and salt in a food processor until combined. Add the butter and pulse until it is in pea-size pieces. Whisk the heavy cream and egg in a small bowl. Pulsing constantly, drizzle the cream mixture into the flour mixture; pulse until the dough comes together. Transfer to a medium bowl and knead 3 or 4 times.
Scoop 6 balls of dough (about 1/2 cup each) onto the prepared baking sheet using a large ice cream scoop or a measuring cup, spacing the dough 2 to 3 inches apart. Refrigerate until firm, about 30 minutes.
Preheat the oven to 400 degrees F. Lightly brush each ball of dough with melted butter and sprinkle with sesame seeds. Bake until lightly golden and a toothpick inserted into the centers comes out clean, 20 to 30 minutes. Let cool 10 minutes on the baking sheet, then transfer to a rack to cool completely.
Meanwhile, make the filling: Toss the plums with the sugar and vanilla bean in a medium saucepan; let sit 30 minutes. Cook over medium heat until the juices are bubbling and syrupy, 10 to 15 minutes. Discard the vanilla bean and transfer the plums to a bowl. Let cool to room temperature, at least 2 hours.
Beat the heavy cream and honey in a medium bowl with a mixer on medium-high speed until medium peaks form. Refrigerate until ready to use.
Split each shortcake and fill with the whipped cream and plums.