Make the homemade toffee: Line a baking sheet with parchment paper. Heat a heavy-bottomed pot over medium heat. Add the granulated sugar, butter, water, and salt. Stir to combine.
Continue to gently stir until the butter melts, about 3 minutes, then stop actively stirring. The mixture will start to bubble. Stir occasionally about once every 2 minutes or so, and let boil until it turns a medium brown color. Then carefully pour the mixture on the baking sheet and set aside to cool completely. Once cooled, use the back of a spoon to break up the toffee into smaller bits.
Preheat the oven to 350F. Line two baking sheets with parchment paper.
Place the butter and brown sugar in a large bowl. Use a stand mixer or hand mixer to mix until combined, about 2 minutes. Add the egg and vanilla extract. Mix until combined, about 1 minute.
Add the flour, graham cracker crumbs, cornstarch, salt, and baking soda. Mix on low until just combined. Add the toffee bits and mix on low until just combined.
Use a large (1/4 cup) cookie scoop to scoop out the dough. Place up to 6 dough balls on each baking sheet. Bake for 9-12 minutes, until the edges are just set. Let the cookies cool on the pan for 10 minutes then transfer to a cooling rack to cool completely.
Once the cookies are cool (you can cool them off in the fridge if you’re in a hurry), pour the heavy cream and sugar into a large bowl. Use a stand mixer or hand mixer to mix just until stiff peaks form.
Add a few slices of banana on each cookie, then top with a scoop of whipped cream. Optional: top with a sprinkle of extra graham cracker crumbs. Enjoy!
