In a small bowl, combine the ground flaxseed with 1 1/2 Tbsp. of water. Mix well, then set aside until thickened, about 5-7 minutes.
Sift together the dry ingredients (flour, baking powder, and salt), then add the sugar and mix well to make sure there are no lumps. Add the cold, cubed butter and mix with your hands until the mixture resembles course crumbs. Add the flax and vanilla and mix well. If this mixture starts to get warm at any point, throw it in the fridge to chill for 5-10 minutes. Just like a pie crust, you want it to stay cold!
Press about 2/3 of the crust mixture into a parchment-lined 8"x8" baking pan. Make sure to really press the mixture into the corners or else the fruit juice could seep down and make the crust soggy. Chill the crust for 15 minutes. While the crust is chilling, preheat the oven to 375ºF.
Once the 15 minutes has passed, bake the bottom crust for 15 minutes until set and slightly golden brown. In a separate bowl, add the chopped pecans to the remaining 1/3 of the crust, then set aside (this will become the top crumble!).