In a medium bowl, stir together graham cracker crumbs and granulated sugar until evenly combined. Pour in melted butter and mix with a fork until crumbs are fully moistened and hold together when pressed.
Press the mixture firmly into a 9 inch pie dish, using the back of a measuring cup to pack it tightly up the sides. Chill in the refrigerator for at least 20 minutes, or bake at 350°F for 8-10 minutes for a crisper base.
In a medium saucepan over medium heat, melt 1/2 cup butter for the filling. Add dark brown sugar and stir constantly until dissolved and bubbling, about 2 minutes.
Pour in sweetened condensed milk and salt, whisking constantly to prevent scorching. Continue cooking over medium-low heat, stirring constantly, until thickened and deep golden caramel color, about 6-8 minutes.
Remove toffee from heat and pour into the cooled crust, spreading evenly with a rubber spatula. Let cool at room temperature for 10 minutes, then refrigerate for at least 1 hour until fully set.
Slice bananas into 1/4-inch rounds and gently toss with lemon juice to prevent browning. Arrange the banana slices in a single even layer on top of the chilled toffee.
In a cold mixing bowl, beat heavy whipping cream, powdered sugar, and vanilla on medium-high speed until stiff peaks form, about 3-4 minutes. Do not overbeat.
Spoon or pipe whipped cream over the banana layer, spreading to the edges or creating decorative swirls. Garnish with shaved dark chocolate if desired.
Return pie to the refrigerator and chill for at least 1 more hour before slicing to allow layers to set together.