Prepare Oreos: If using whole Oreos, separate the cookies and scrape off the white filling with a knife or your finger (the filling can be saved for other desserts). Keep only the black cookie parts. Place the black cookies in a food processor and blend into fine crumbs. (Alternatively, place cookies in a sealed bag and crush thoroughly with a rolling pin until no large pieces remain).

Mix Butter & Crumbs: Place 70g of unsalted butter in a small bowl and melt completely by heating over a double boiler (or microwave on medium-high for 20-30 seconds). Pour the melted butter into the Oreo crumbs. Quickly mix with a silicone spatula until every crumb is evenly coated with butter (The mixture should hold together when pressed but crumble easily).

Press into Mold: Transfer the buttered crumbs into a 6-inch springform pan. Use the flat bottom of a measuring cup, glass, or spatula to press the crumbs firmly and evenly into the bottom of the pan (Pay special attention to the edges to prevent crumbling later). Place the pan in the refrigerator to chill while you prepare the mousse layer.

Soften Cream Cheese: Place the 250g of softened cream cheese in a large bowl. Add 20g of fine sugar and mix with a spatula until the cheese is smooth and free of lumps (If small lumps remain, place the bowl over warm water ~40°C / 104°F and stir to help melt them).

Combine Base Ingredients: Add 150g of heavy cream and 100g of blueberry jam to the cream cheese mixture. Continue stirring for about 30 seconds until the jam is evenly distributed (The mixture will turn light purple and smell like blueberries).

Add Gelatin: Pour 50g of milk into a small bowl. Heat in the microwave for 30 seconds (or over warm water) until warm (the bowl should feel warm, not hot, to the touch). Add the softened, squeezed-dry gelatin sheets. Stir quickly with a spoon or chopsticks until the gelatin is completely dissolved (no transparent bits remaining). Slowly pour the gelatin-milk liquid into the blueberry cheese mixture while stirring constantly. Ensure it is fully incorporated to prevent the gelatin from cooling and clumping in one spot, which would affect the texture.

Strain for Smoothness (Optional): For an ultra-smooth mousse texture, strain the prepared mousse mixture through a fine-mesh sieve to remove any potential leftover cheese lumps or berry seeds. This step is optional and does not affect the overall flavor.

Pour Mousse: Take the chilled Oreo base out of the refrigerator. Slowly pour the prepared (and optionally strained) mousse mixture into the pan. Lift the pan and gently tap it vertically on the counter 3-5 times to release any air bubbles trapped in the mousse (bubbles can affect the final appearance).

Chill to Set: Place the pan in the refrigerator (set to 2-4°C / 36-39°F). Let it chill for at least 4 hours (overnight is ideal for a firmer set and easier, cleaner unmolding). Avoid frequently opening the refrigerator door during this time to prevent temperature fluctuations that could cause uneven setting.

Unmold: Once fully set, remove the pan from the refrigerator. Wrap a hot towel (wrung out well) around the outside of the pan for 10-15 seconds (the heat helps slightly expand the metal, loosening the mousse from the edges for easier release). If using a springform pan, simply release the clasp and gently lift the ring. If using a solid pan, carefully run a thin knife around the edge before inverting onto a serving plate.

Decorate: Use a spatula to evenly spread the additional 20-30g of blueberry jam over the top surface of the mousse. Rinse the fresh blueberries and pat them completely dry. Arrange them on top of the mousse as desired (in a circle, line, or pattern). You can also add mint leaves for a fresh touch.