Line 1 large tray, or 2-3 smaller trays, with parchment or silicone baking mats.

In a medium bowl, whisk together all-purpose flour, cocoa powder, and salt. Set aside.

In a large mixing bowl, whip butter on medium-high speed until light and fluffy. Add the caster sugar and mix on medium-high speed until creamy. Add the egg and vanilla bean paste and mix until fully incorporated.

Add the dry ingredients and mix on medium speed until a clumpy dough forms, pausing to scrape down the sides of the bowl as necessary.

Use your hands to finish bringing the mixture together to form a smooth, slightly sticky dough.

Form dough into a 7-inch disc. Wrap tightly in plastic wrap and refrigerate at least 1 hour, or until firm enough to roll.

On a lightly floured countertop or silicone rolling mat, roll chilled dough to about 5 millimeters thick.

With a 1-inch x 2 1/2-inch rectangular cutter, cut into rectangles. Re-roll scraps and cut remaining biscuits. (You should have about 36-40 rectangles total.)

Transfer to prepared trays, allowing about 1/2-inch space between the biscuits.

Refrigerate trays for 30 minutes. Meanwhile, preheat oven to 355° F /180° C. Bake in preheated oven, on tray at a time, for 10-11 minutes. Do not overbake.

Cool 5 minutes on trays, then transfer to a wire rack to cool completely. Meanwhile, prepare the chocolate cream.

In a large mixing bowl, whip butter on high speed until smooth and creamy, about 30 seconds. Add powdered sugar, cocoa powder, and kosher salt, and mix until smooth and creamy, gradually drizzling in the milk (you might not use all of it). Transfer to a piping bag fitted with a 1/2-inch round tip.

Divide biscuits into pairs. Pipe cream on the flat side of half of the biscuits. Sandwich with the second biscuit.

Store bourbon biscuits in an airtight container at room temperature for up to five days.