In a large mixing bowl, combine heavy whipping cream and icing sugar, then beat using an electric mixer on medium speed until soft peaks form (about 3-5 minutes).

Gently warm the creamy peanut butter in the microwave for 15-20 seconds, then fold it into the whipped cream mixture until stiff peaks form.

Line a 9x5 loaf pan with cling wrap or parchment paper, leaving an overhang on the sides for easy removal.

Dip each oatmeal cookie into whole milk briefly to soften, then layer them in the prepared loaf pan, pressing gently to ensure they fit snugly.

Spoon one-third of the peanut butter whipped cream over the first cookie layer, then sprinkle mini semi-sweet chocolate chips and mini M&Ms over the cream.

Repeat layering two more times, finishing with a final layer of peanut butter whipped cream on top.

Cover the assembled cake with plastic wrap and chill in the refrigerator for at least 4 hours, or freeze for about 2 hours.

When ready to serve, gently lift the cake out of the pan using the overhanging cling wrap or parchment paper and slice into thick pieces.