01 - Combine mascarpone cheese, lemon zest, lemon juice, sugar, and vanilla extract in a large mixing bowl. Beat with hand mixer on medium speed until completely smooth and well blended, approximately 2 minutes.
02 - In a separate chilled bowl, whip cold heavy cream using hand mixer or stand mixer until stiff peaks form. Be careful not to overwhip.
03 - Gently fold whipped cream into mascarpone mixture in two separate additions using a rubber spatula. Use folding motion to maintain airy texture until fully incorporated and no white streaks remain.
04 - Divide mousse evenly among 4 serving glasses or bowls using a spoon or piping bag for elegant presentation. Smooth tops with spatula.
05 - Cover servings with plastic wrap or lids. Refrigerate for minimum 2 hours until mousse is fully set and thoroughly chilled.
06 - Just before serving, top each portion with fresh berries, sprinkle of additional lemon zest, and fresh mint leaves if desired.