In a medium saucepan over low heat, combine the softened butter, marshmallows, cream cheese, and white chocolate. Stir gently until the mixture is completely smooth and melted, about 5–7 minutes. Remove from heat and mix in 6 tablespoons of strawberry jam, allowing to cool at room temperature while preparing the next step.

In a large mixing bowl, pour in the chilled heavy whipping cream. Using an electric mixer, beat on medium speed until stiff peaks form, about 3–4 minutes. Gently fold the whipped cream into the cooled strawberry cheesecake mixture until no white streaks remain. Refrigerate this filling for at least 30 minutes.

Once the filling has chilled and thickened, take your tart shells and place them on a serving tray. Using a piping bag or a ziplock bag with a corner snipped, pipe the creamy filling into each tart shell. Add a dollop of extra strawberry jam in the center of each tart.

In a clean mixing bowl, combine mascarpone cheese, powdered sugar, and remaining heavy cream. Beat until stiff peaks form, about 2–3 minutes. Scoop generous dollops onto each tartlet, giving them a beautiful, swirled appearance.

Use melted chocolate or fondant to create Miffy’s facial features on each tartlet. Pipe the chocolate for details or use edible markers for a quick approach.