For the crust:
Prepare ice water in a small bowl and set aside.
In a large bowl, whisk together the flour, sugar, and salt.
Use a pastry cutter or fork to cut in the butter and Crisco until the mixture resembles coarse crumbs with pea-sized butter pieces.
Beat one egg in a small bowl, pour out half, and mix the remaining half with 1/4 cup ice water. Add to the flour mixture and stir until just combined.
Divide the dough in half, shape one half into a disc, and save the other half for later. Roll the dough out on a floured surface, then transfer to a 9-inch pie pan. Fold any excess crust under, crimp, and place in the fridge or freezer.
For the filling:
Preheat oven to 400°F (200°C) with a baking sheet on a center rack.
In a saucepan, melt the butter over medium heat until it turns golden brown with a nutty aroma, about 5 minutes. Remove from heat and let cool slightly.
In a large bowl or stand mixer, beat the eggs for 2 minutes. Add the granulated sugar, brown sugar, and corn syrup, and beat for another minute.
Stir in the browned butter, milk, vanilla, salt, lemon juice, and flour until smooth. Fold in the toffee bits and chopped pecans.
Pour the filling into the prepared pie crust and place the pie on the hot baking sheet in the oven. Reduce the oven temperature to 350°F (175°C) and bake for 30 minutes.
For the streusel:
While the pie bakes, whisk together the flour and brown sugar in a medium bowl. Use a pastry cutter to cut in the cold butter until the mixture resembles coarse sand. Chill the streusel until ready to use.
After 30 minutes of baking, remove the pie from the oven, add the streusel topping, and cover the crust with a pie shield.
Continue baking at 350°F for another 30-37 minutes. The pie should be slightly jiggly in the center when done. Remove and cool on a rack before slicing.