Line the bottom and the sides of a 7-inch springform pan with parchment paper or use a 9-inch pie plate.
For the crust, melt the butter in a small saucepan. Add the crumbs and stir to coat them.
Press the mixture into the bottom and up the sides of the pan.Use a measuring cup to pat down the crumbs. Refrigerate until the filling is ready.
In the bowl of a stand mixer fitted with the paddle attachment beat mascarpone cheese at low-medium speed until smooth, about 1 minute.
Add the sweetened condensed milk slowly and beat into mascarpone until smooth and well mixed.
Add the heavy cream and mix until combined.
Slowly add the lemon juice to the mixture while beating at low-medium speed. The mixture will start to thicken and gets creamy.
Pour into prepared pan and refrigerate for at least 6 hours or overnight until firm.