In a medium saucepan, combine the fresh or frozen strawberries, granulated sugar, and water. Place over medium heat and bring to a simmer, stirring occasionally, for 8–10 minutes until the berries break down and the liquid thickens slightly.
Pour the hot mixture through a fine-mesh sieve into a bowl, pressing on the solids with a spatula to extract all the juice.
Measure out ⅓ cup of this strained sauce and set it aside to cool completely (this goes into the cream). Keep the remaining sauce in a shallow dish to cool to room temperature (this is your dipping liquid).
In a large mixing bowl, add the room-temperature mascarpone cheese, the ⅓ cup of strawberry sauce, granulated sugar, and vanilla extract. Whisk with an electric hand mixer until the base is perfectly smooth and uniform in color.
Pour the cold heavy cream directly into the same bowl. Start your mixer on low speed to incorporate the liquid, then increase to medium-high. Whip the entire mixture together until it reaches thick, stiff peaks that hold their shape.
Assemble and top
Briefly dip your ladyfingers into the room-temperature strawberry dipping sauce (about 1–2 seconds per side). Arrange them in a tight, single layer along the bottom of your baking pan.
Spread exactly half of your prepared strawberry mascarpone cream over the soaked ladyfingers, smoothing it out evenly with an offset spatula.
Add a second layer of ladyfingers quickly dipped in the strawberry sauce, then top with the remaining half of the mascarpone cream. Smooth out the surface.
Cover the pan tightly with plastic wrap and refrigerate for at least 4 hours (or overnight) to let the ladyfingers soften.
Right before serving, dust the top generously with your ground freeze-dried strawberry powder and top with fresh sliced strawberries if desired.
Notes
Cool the Sauce: Never add warm strawberry sauce to your cheese or cream. Warm sauce will melt the cream and make your filling runny.
Soft Cheese, Cold Cream: Your mascarpone must be at room temperature so it does not form lumps. Your heavy cream must be ice-cold so it whips up thick.
Do Not Overmix: Watch the bowl closely when whipping the cream. If you mix mascarpone too long, it will separate and look grainy. Stop mixing as soon as it looks thick.
Dip Quickly: Only dip the ladyfingers in the sauce for 1 second per side. If they soak too long, they will get soggy and watery.
Press the Fruit: When straining your sauce, press hard on the berries. The best flavor is in the thick part of the fruit!