Prepare the pan: Line an 8x8-inch (20x20 cm) pan with parchment paper, leaving overhang on two sides for easy lifting.
Crush the biscuits: Pulse digestive biscuits in a food processor until fine crumbs form, or crush in a zip-top bag with a rolling pin. Measure 2 ½ cups (250 g).
Toast the oats (optional): Spread rolled oats on a baking sheet and toast at 350°F (175°C) for 5-7 minutes until fragrant. Cool completely.
Combine dry ingredients: In a large bowl, mix crushed biscuits, oats, salt, and chia or flax seeds until evenly distributed.
Prepare wet binders: Warm condensed milk, peanut butter, honey, and vanilla extract in a microwave-safe bowl for 30-45 seconds, or in a saucepan on low heat for 2 minutes, until smooth and pourable.
Mix the dough: Pour the warm wet mixture over the dry ingredients. Fold and press with a spatula until no dry spots remain. The dough should feel thick and slightly sticky.
Add mix-ins: Fold in chocolate chips, dried fruit, and nuts, distributing evenly without overmixing.
Press into pan: Transfer the mixture to the prepared pan. Press firmly and evenly using the bottom of a measuring cup, aiming for a uniform thickness of about ¾ inch (2 cm).
Chill: Refrigerate for at least 1 hour, or freeze for 20-30 minutes for faster setting.
Slice: Lift the bars out using the parchment overhang. Slice into 16 equal squares with a sharp knife, wiping the blade between cuts for clean edges.