Line baking pan with parchment paper in an 8x8 inch square pan, leaving overhang on two sides. The overhang creates handles for easy lifting after fudge sets. Press parchment firmly into corners.

Melt butter and milk in a heavy-bottomed saucepan over medium-low heat, stirring constantly. Cook for 3-5 minutes until completely smooth and combined. Don't rush this step or mixture might separate.

Add white chocolate chips to the warm butter mixture and reduce heat to low. Stir constantly for 5-7 minutes until chips melt completely. Scrape bottom and sides frequently to prevent scorching.

Remove from heat immediately once chocolate melts fully to prevent overcooking. The residual heat continues cooking, so timing here is crucial for smooth texture.

Stir in cookie butter along with vanilla extract and salt while mixture is still hot. Mix vigorously until completely smooth and uniform in color. No streaks should remain.

Pour into prepared pan immediately before mixture cools and thickens. Use a spatula to scrape every bit from the saucepan. Spread evenly into corners and smooth the top.

Crush Biscoff cookies by placing them in a zip-top bag and crushing with a rolling pin. Create a mix of fine crumbs and larger pieces for textural interest.

Sprinkle cookie crumbs generously over warm fudge surface while it's still soft. Press down gently with the back of a spatula so they adhere as fudge sets.

Refrigerate for minimum 2-3 hours or preferably overnight until completely firm and sliceable. Don't try cutting too early or fudge will be too soft and sticky.

Cut into squares by lifting fudge out using parchment handles. Use a sharp knife dipped in hot water and wiped dry between cuts for clean edges. Cut into 1-inch squares.