In a small stainless steel bowl, sprinkle the gelatin over the water. Set aside to bloom for about 5 minutes.
In a medium bowl, whisk the mascarpone with the sugar, 1 cup of the cream, lemon zest, and vanilla. Set aside.
Add the final ¼ cup of cream to the bloomed gelatin, and set the bowl over a pan of simmering water. Heat the gelatin/cream mixture until the gelatin has dissolved and you can no longer see the gelatin flakes, stirring constantly.
Take the gelatin off the heat and stir it into the mascarpone mixture.
Pour the panna cotta into 4 glasses or ramekins.
Transfer to the fridge and chill to set, about 3 hours, if not overnight.
Lemon jelly
Pour ¼ cup of lemon juice in a 2-cup measurer or a medium bowl. Sprinkle the gelatin over top. Set aside to bloom at least 5 minutes.
In a small saucepan, bring the water and sugar to a boil to completely dissolve the sugar.
Pour the syrup over the gelatin, and whisk to dissolve the gelatin.
Add the remaining ¼ cup of lemon juice. Let the lemon jelly cool to room temperature before using.
Assembly
When the panna cottas are set, and the lemon jelly has cooled to room temperature, pour the lemon jelly over top each of the panna cotta. Pour as little or as much as you would like (I poured a 1-inch layer on mine, but I think a half-inch would have been better).
Chill in the fridge to set the jelly, about 1 hour.
Crumble 2 of the florentine cookies over each of the panna cottas before serving.
Notes
Panna cotta loosely based on this recipe featured in the Toronto Star newspaper
Given the acidity of the lemon juice, I used a full packet of gelatin so that it would set properly.