Preheat the oven to 375°F. Butter or spray with cooking spray a 13 x 9-inch baking pan and line with parchment paper hanging over the sides; set aside.
In a large bowl, whisk together flour, sugar, baking powder, and salt. Using a pastry cutter or two forks, cut the cold cubed butter into the flour mixture until it resembles coarse crumbs. Add the egg yolks and cold water; stir it in until fully combined (you may also use the pastry cutter to help blend it in). Press half of the crumble mixture into the prepared pan, going up the sides a little, for the crust; set aside the other half for the topping.
In a large bowl, stir together the peach slices, sugar, flour, lemon zest and juice, vanilla, cinnamon, and nutmeg until just combined. Evenly distribute the peach mixture on top of the prepared crust. Sprinkle the remaining half of the crumble mixture over the top of the peaches.
Bake for 50-60 minutes, until the top is golden brown. Cool the crumble bars to room temperature. Remove from the pan by lifting up with the edges of parchment paper. Cut in 3-lengthwise and 5-crosswise squares to make 15 servings.
Serve with a scoop of vanilla bean ice cream or whipped cream on top.