Step 1: In a large mixing bowl, beat the softened cream cheese with an electric mixer on medium speed until completely smooth and free of lumps. Scrape down the sides of the bowl, then add the powdered sugar, 1/4 cup granulated sugar, vanilla extract, and fresh lemon juice. Beat again until well combined and smooth.

Step 2: In a separate, chilled bowl, whip the cold heavy whipping cream on high speed until stiff peaks form. Gently fold about one-third of the whipped cream into the cream cheese mixture to lighten it. Then, add the remaining whipped cream and fold carefully until just combined, being careful not to overmix. Cover and refrigerate for at least 30 minutes.

Step 3: Preheat your oven to 400°F (200°C). In a small bowl, mix together the 1/2 cup granulated sugar and ground cinnamon. Brush both sides of each flour tortilla with melted butter, then sprinkle generously with the cinnamon-sugar mixture.

Step 4: To form the shells, drape the prepared tortillas over two rungs of your oven rack or place them in the spaces of an inverted muffin tin. Bake for 6-8 minutes, or until golden brown and crispy. Watch them closely to prevent burning. Let them cool completely on a wire rack.

Step 5: Once the shells are cool, remove the cheesecake filling from the refrigerator. Spoon or pipe the filling into each taco shell. Top generously with fresh mixed berries and serve immediately.