1-In a small bowl, mix the dulce: In a small bowl, mix the dulce de leche for about a minute to soften. In a separate bowl, whip the heavy cream until stiff peaks form, being careful not to over-mix. Fold the softened dulce de leche into the whipped cream until smooth.
2-Chop, break, or grate the peppermint crisp: Chop, break, or grate the peppermint crisp bars into small pieces. (A couple of pulses in a food processor works well too.) Fold half of the peppermint crisp pieces into the cream mixture and set the rest aside.
3-Line the bottom of a square or: Line the bottom of a square or rectangular baking dish (such as 7″ × 10″) with tennis biscuits. Spread half of the cream mixture evenly over the biscuits.
4-Add another layer of tennis biscuits, followed: Add another layer of tennis biscuits, followed by the remaining cream mixture. Spread evenly with an offset spatula.
5-Sprinkle the remaining peppermint crisp over the: Sprinkle the remaining peppermint crisp over the top and refrigerate for at least 2 hours before serving.