Make the espresso mascarpone base: In a separate medium bowl, combine the softened mascarpone cheese, remaining powdered sweetener, cooled espresso, vanilla extract, and sea salt. Beat with the mixer on medium speed until smooth, creamy, and fully combined. The mixture should be silky with no lumps of mascarpone.
Fold the whipped cream into the mascarpone mixture: Add about one-third of the whipped cream to the espresso mascarpone mixture and gently fold it in with a spatula to lighten the base. Once mostly incorporated, add the remaining whipped cream and continue to fold, turning the bowl as you go, just until no streaks remain. You should now have one unified, airy, tiramisu-style creamy mixture.
Layer the creamy mixture over the berries: Using a spatula, dollop the creamy mixture evenly over the mixed berries in the glass casserole dish. Gently spread it to the edges, taking care not to disturb the berries too much. The goal is a visible, distinct layer of berries on the bottom and one smooth, generous layer of cream on top, just as you would for a classic tiramisu.
Chill to set the flavors: Cover the casserole dish tightly with plastic wrap or a lid and refrigerate for at least 2 hours, or up to overnight. This resting time allows the flavors of the espresso and mascarpone to develop and the cream to firm up slightly, so each spoonful feels luxurious rather than loose.
Finish and serve: Just before serving, if using, sift the cocoa powder evenly over the top of the chilled cream layer for a nod to traditional tiramisu. Scatter the extra mixed berries over the surface for color and freshness. Use a large spoon to scoop out parfait-style portions, making sure each serving includes both creamy tiramisu layer and berries from the bottom.
