Chill your bowl and beaters for whipping cream.
Line a baking sheet with paper towels to cool the puree quickly.
Cook the strawberry puree
Hull and chop 1 1/2 pounds strawberries (about 4 cups chopped).
Add to a saucepan with 1/3–1/2 cup sugar, 1 tablespoon lemon juice, and a pinch of salt.
Simmer over medium heat for 8–10 minutes, stirring, until juicy and softened.
Blend until smooth, then return to the pan and cook 5–8 minutes more to reduce to about 1 cup. You want a thick, spoon-coating consistency.
Spread on the lined sheet to cool to room temperature. Do not add hot puree to cream.
Bloom the gelatin
Sprinkle 2 1/4 teaspoons unflavored gelatin over 3 tablespoons cold water in a small bowl. Let stand 5–10 minutes until spongy.
Melt the gelatin
Microwave the bloomed gelatin for 8–10 seconds until just melted and clear. Do not boil.
Whisk 2 tablespoons of the cooled strawberry puree into the melted gelatin to temper it.
Combine with strawberry base
Whisk the tempered gelatin mixture into the remaining cooled puree until fully blended. Set aside; it should remain pourable but not warm.
Whip the cream
In the chilled bowl, whip 1 1/2 cups heavy cream with 1 teaspoon vanilla and 1–2 tablespoons powdered sugar (optional) to medium peaks.
Stop when the whisk leaves defined ribbons and peaks gently hold their shape.
Fold it all together
Stir one-third of the whipped cream into the strawberry mixture to lighten.
Gently fold in the remaining cream with a spatula until no streaks remain. Work slowly to keep the mousse airy.
Chill to set
Cover and refrigerate 30–45 minutes until softly set and spreadable. For a firmer fill, chill 1–2 hours.
Create a buttercream dam: Pipe a ring of buttercream around the cake edge to prevent bulging.
Use chilled cake layers: Cool layers grip the filling and help it set quickly.
Measure thickness: Aim for 1/2–3/4 inch of filling per layer for stability.
