Center a rack in the oven and preheat to 350 (F).
In a medium bowl, combine 3 cups all-purpose flour, 1 teaspoon baking powder, and 1 teaspoon kosher salt. Set aside.
In the bowl of a freestanding electric mixer fitted with a paddle attachment, combine 2 1/2 cups granulated sugar and the zest from 3 medium-large lemons. Use your fingers to rub the zest into the sugar until the zest is fully incorporated throughout and the sugar smells fragrant. Add 1 cup unsalted butter, and beat on medium-high for at least 5 minutes, until the mixture is light, fluffy, and has doubled in volume.
Once the batter is light and fluffy, reduce the mixer speed to its slowest setting. Add 4 large eggs, one at a time, only adding the next egg when the one before it has fully incorporated into the batter. Add 1/2 teaspoon lemon oil or extract.
Once all the eggs and the extract are added, keep the mixer on its slowest setting. Add the dry ingredients (from the 2nd step) in three parts, alternating with 1 cup sour cream in two parts, beginning and ending with the dry ingredients. Continue mixing just until the dry ingredients have incorporated into the batter — BE CAREFUL NOT TO OVERMIX, or your cake will be dry and dense and I will cry for you.
Spray a 10-inch bundt pan generously—and I mean, GENEROUSLY—with cooking spray. Make sure you spray entirely up the sides and the weird circle thing in the middle so you don’t have a hard time unmolding the cake! Immediately transfer the batter into the bundt pan, using an offset spatula to spread the batter evenly throughout the pan and smoothen out the top. Bake in the preheated oven for 55 to 60 minutes, or until a skewer inserted into the center of the cake comes out clean and the cake bounces back when gently poked. Transfer to a wire rack, and use a wooden skewer to poke several deep holes across the bottom of the cake. Use a pastry brush to brush 3/4 cup of the lemon simple syrup across the bottom of the cake and into the holes, pausing every so often to allow the syrup to seep into the holes. Allow the cake to cool in the pan on a wire rack for 30 minutes, before inverting it out onto a cake plate so that the glazed portion is the bottom of the cake. Brush the top of the cake with the remaining lemon syrup and allow to cool completely to room temperature before glazing.