Butter and line an 8X8 inch pan with parchment paper, set aside.

In a large microwave-safe bowl add the chocolate, sweetened condensed milk, and butter. Microwave for 2 minutes or until the chocolate is melted. This timing is a based on my microwave which is 1200 Watts, so your cook time may very. (You can also make the fudge over a bain-marie)

Once melted, stir to combine. Do not be tempted to over-mix the fudge as this can cause it to separate.

Fold in the cookie butter reserving some to swirl over the top.

Transfer the fudge to the prepared tin, smooth out the top, then spoon over the reserved cookie butter. Using a butter knife or tooth pick, swirl the cookie butter across the top of the fudge. You'll want to work quickly doing this before the fudge begins to set.

Allow to set in the fridge for a minimum of 2 hours to overnight.

Remove from the fridge and cut into 16 squares. Cover and store in an airtight container for up to 4 weeks.