Line a rimmed baking pan or pans (for example a rimmed baking sheet or multiple 8×8‑inch pans) with aluminum foil, leaving an overhang on two opposite sides for lifting. Set aside.
Place the entire 11 lb container Pillsbury vanilla frosting and the 2 3/4 cups Biscoff Spread into a very large microwave‑safe bowl. If the full amount will not fit in one bowl or in your microwave, divide the ingredients into batches that do fit.
Microwave on high in 30‑second intervals, removing and stirring thoroughly after each interval. Continue heating and stirring until the Biscoff is melted and the mixture is smooth and fully combined.
Pour or spoon the warm mixture into the prepared foil‑lined pan(s) and spread evenly to the edges with a spatula.
Refrigerate until the fudge is completely firm, at least 1–2 hours (longer or overnight if needed).
Use the foil overhang to lift the set fudge from the pan(s). Peel back the foil and cut the fudge into pieces. Store refrigerated.
Notes
If the ingredients won't fit in one bowl or microwave, divide into batches.
Store refrigerated.