Line an 8×8-inch square pan with parchment paper, leaving overhang for easy removal.

Make the crumble topping:

Mix flour, brown sugar, melted butter, and cinnamon until crumbly.

Bake at 350°F (175°C) for 8–10 minutes until lightly golden.

Cool completely.

In a saucepan over low heat, melt white chocolate chips, sweetened condensed milk, and butter, stirring constantly until smooth and glossy.

Remove from heat and stir in vanilla extract and salt until fully combined.

Stir lemon zest into the blueberry preserves.

Pour half of the fudge into the prepared pan and smooth evenly.

Spoon half of the blueberry mixture over the fudge and gently swirl.

Add remaining fudge, then dollop and lightly swirl the remaining blueberry mixture.

Sprinkle the cooled muffin crumble evenly over the top, pressing gently.

Add freeze-dried blueberries if using.

Refrigerate for 2–3 hours, or until fully set.

Lift out using parchment and slice into squares with a clean, sharp knife.