Step 1: Preheat your oven to 350°F (175°C). Lightly grease a 9-inch tart pan with a removable bottom.
Step 2: For the Biscoff crust, combine the crushed Biscoff cookies, melted unsalted butter, and 1 tbsp granulated sugar in a medium bowl. Mix until well combined and the crumbs are moistened.
Step 3: Press the Biscoff mixture firmly and evenly into the bottom and up the sides of the prepared tart pan. Bake for 10 minutes, then remove from the oven and let it cool slightly while you prepare the filling.
Step 4: Reduce the oven temperature to 325°F (160°C).
Step 5: For the lemon filling, in a heatproof bowl set over a saucepan of simmering water (or a double boiler), whisk together the egg yolks, whole egg, lemon zest, 1/2 cup freshly squeezed lemon juice, and 1/2 cup granulated sugar. Do not let the bowl touch the water.
Step 6: Continuously whisk the mixture for 8-10 minutes, or until it thickens enough to coat the back of a spoon. Be careful not to scramble the eggs.
Step 7: Remove the bowl from heat. Gradually whisk in the cubed unsalted butter, one cube at a time, until it is fully melted and incorporated, and the filling is smooth and glossy.
Step 8: Pour the warm lemon filling into the cooled Biscoff crust. Gently tap the tart pan on the counter a few times to release any air bubbles.
Step 9: Bake for 20-25 minutes, or until the edges are set and the center has a slight wobble when gently shaken. Do not overbake.
Step 10: Carefully remove the tart from the oven and let it cool completely on a wire rack. Once at room temperature, transfer the tart to the refrigerator and chill for at least 4 hours, or preferably overnight, before serving. This allows the filling to fully set.