Cut butter into chunks and add back into the refrigerator. In a large bowl or food processor, combine flour, cinnamon, nutmeg, sugar, and salt. Whisk together.
Add chunks of butter into the flour mixture and pinch together until butter is fully incorporated into the flour and pea-sized morsels form. (If using a food processor, pulse until butter and flour come together to form pea-sized morsels).
Make a well in the center of the bowl and begin adding ice cold water a 1-2 tablespoons at a time. Mix until a shaggy dough forms.
Turn dough out onto a lightly floured work surface and press any scraps together to form one cohesive dough. Wrap in plastic wrap and place in the refrigerator for one hour.
While the pie dough is resting in the refrigerator, peel and halve the sweet potatoes. Boil the sweet potatoes in a large pot until fork tender. In a separate pan, brown the butter. Set aside and allow it to cool.
Once the sweet potatoes have finished cooking, drain and add to a large mixing bowl. Mash the sweet potatoes and add the brown butter, brown sugar, sugar, spices, and evaporated milk. Mix together until combined and allow them to cool completely. Be sure to remove any stringy fibers from the sweet potato flesh. (Optional step: press sweet potato mixture through a fine mesh sieve to remove any lumps and residual stringy fibers)
Then add in vanilla extract, flour, and eggs and mash together until eggs are fully incorporated.
Preheat the oven to 425 degrees F. Roll the pie dough out into a 10-inch circle until it's about a ¼ inch thick.
Place pie dough into a pie pan and trim off any excess pieces that don't fit into the pan. If you'd like to crimp the pie crust edges, do so here. Freeze the pie for 20 minutes.
Polk holes in the bottom of the crust so that air bubbles do not form while baking. Add parchment paper to the pie pan and add pie weights to the unbaked pie crust and place in the preheated oven for 17-20 minutes, until the edges of the crust is golden brown.
Remove the pie weights from the pie shell and continue to bake for another 12-13 minutes, until the bottom of the pie dough is golden brown. Add the sweet potato filling. Reduce the oven temperature to 350 degrees F and bake for 35-40 minutes, until the center of the pie is slightly jiggly. Allow your baked sweet potato pie to cool completely on a wire rack.
(Tip: add the meringue topping only after the pie has cooled completely. If the pie is warm, the meringue will melt) To make the brown sugar meringue topping, use a hand mixer / electric mixer to whip the egg whites. Start on medium speed then increase to high speed and add the cream of tartar. Continue to mix for another minute then slowly begin to add brown sugar. Whip until glossy and then add vanilla extract.
Mix together add to a piping bag and pipe on top of the pie once the pie has cooled completely. Pro tip - use a torch to brown the meringue for those gorgeous browned streaks!