Melt 6 tablespoons butter in medium skillet over medium-high heat.
Whisk in brown sugar.
Whisk until mixture bubbles thickly, about 2 minutes.
Mix in cracker crumbs; stir 1 minute. Using back of fork, press warm crumb mixture over bottom and up sides of 9-inch-diameter tart pan with removable bottom. Cool crust completely.
Pour milk into small custard cup.
Sprinkle gelatin over.
Let stand until gelatin softens, about 15 minutes.
Whisk 1 cup sugar, lemon juice, and lemon peel in heavy medium saucepan to blend well.
Whisk in eggs, then remaining 4 tablespoons butter.
Whisk over medium heat until custard thickens and just begins to boil, about 5 minutes; remove from heat.
Add gelatin mixture; whisk until gelatin dissolves.
Let stand until beginning to set, about 10 minutes.
Pour warm filling into crust. Chill until filling is firm, at least 3 hours and up to 1 day.
Beat cream and remaining 1 tablespoon sugar in medium bowl until peaks form. Pipe or spoon cream over tart.