Pulse the biscoff cookies in a food processor until you have about 2 cups of finely ground crumbs.
In a large bowl, combine the cookie crumbs, granulated sugar, and melted butter. Mix until it resembles wet sand.
Press the mixture into the bottom of the tart pan and slightly up the sides. Make sure it is tight and compact! You can use the bottom of a flat measuring cup to help pack down the crust.
Bake the crust at 350°F for about 15 minutes or until it's dried out.
Let the crust fully cool to room temperature before filling.
Cookie Butter Filling
In a large bowl using a handheld mixer or stand mixer fitted with a paddle attachment, cream the room temperature butter and cookie butter together until smooth.
Add in the powdered sugar and salt and mix until fully incorporated.
Add in 2-4 tablespoons of heavy cream if needed to thin out the frosting. Then beat the frosting on high for 4-5 minutes until light and fluffy!
Spoon the cookie butter filling into the cooled biscoff tart crust and smooth into and even layer. Place in the fridge to chill for at least an hour to firm up.
Finely chop the dark chocolate and place into a large bowl.
Place the heavy cream in a microwave safe bowl and microwave for 1-2 minutes or until hot to the touch but not boiling.
Pour over the chopped chocolate and let sit for about two minutes.
Using a spatula, gently mix until the chocolate is fully melted and the ganache is fully combined and smooth.
Let sit at room temperature or in the fridge until is has cooled to the touch. [This is important so that it doesn't melt the cookie butter filling!]
Pour the cooled ganache over the cookie butter frosting layer and smooth into a smooth, even layer.
For the feathered topping, microwave a little bit of cookie butter for 10-20 seconds. Then let it cool slightly so it's cool to the touch but stilly melty enough to pipe.
Add the melted cookie butter to a piping bag (or plastic sandwich bag) and cut a small hole in the tip. Then pipe horizontal stripes of melted cookie butter across the dark chocolate ganache. Then feather using a knife or skewer by gently dragging it up and down the surface of the chocolate, perpendicular to the lines of piped cookie butter.
Chill the chocolate biscoff tart in the fridge for at least 1 hour or until the chocolate is fully set.
If desired, top with flaky sea salt! I just love the combo of sea salt and dark chocolate, but you can leave it off if you prefer.
This recipe can be used to make a few different sizes of cookie butter tart!