In a medium saucepan, combine half of the strawberries, sugar, Cointreau (if using), and a pinch of salt. Cook over medium heat, stirring until the sugar dissolves. Cover and simmer for about 10 minutes, stirring occasionally, until the strawberries break down into a juicy sauce.
1½ pounds fresh strawberries, 2 tablespoons sugar, 1 tablespoon Cointreau, 1 Pinch sea salt
Strain the mixture through a fine-mesh sieve into a heatproof bowl to remove solids. Stir in the water, then refrigerate until chilled.
½ cup water
In a large bowl, mix mascarpone and vanilla until smooth.
8 ounces mascarpone cheese, ½ teaspoon vanilla extract
In a separate bowl, beat the cream and powdered sugar with a mixer on medium speed for 5–8 minutes, or until soft to medium peaks form. Gently fold the whipped cream into the mascarpone mixture until just combined.
1 cup heavy whipping cream, ½ cup powdered sugar
Quickly dip half the ladyfingers into the chilled strawberry syrup, coating them on all sides. Arrange in a single layer in a 9x9-inch (or similar) dish, trimming if needed.
20–24 ladyfingers
Spread half the mascarpone mixture evenly over the ladyfingers. Top with half the fresh sliced strawberries. Repeat with the remaining ladyfingers, mascarpone mixture, and strawberries. Cover and refrigerate for at least 8 hours or overnight for best flavor.
Just before serving, top with extra strawberries and a dusting of powdered sugar.