Preheat oven to 350°F. Combine the cookie crumbs, butter, and salt in a 9-inch fluted tart pan. Press into an even layer on the bottom and sides of the pan. Bake crust until light golden brown and fragrant, 10 to 12 minutes. Transfer to a rack to cool completely.
Set a fine mesh sieve over a medium bowl. In another medium bowl, whisk together the egg yolks, sugar, cornstarch and salt. In a medium saucepan, heat the milk and cream over medium until hot. Very gradually whisk the hot milk into the yolk mixture, then return the mixture to the saucepan. Stirring constantly, cook the custard over medium heat until it comes to a very slow boil. Continue cooking another 2 minutes until the custard is very thick, then immediately strain the custard through the sieve. Stir in the Cointreau and butter until melted. Pour the custard into the prepared crust. Cover with plastic film and chill until completely cold, at least 4 hours.
Just before serving, toss the peaches with the lemon juice and the sugar. Top the tart with the peaches.