Heat oven to 350° F. Spray an 8-inch square baking pan with the cooking spray. Line the pan with 2 crisscrossed pieces of parchment, leaving an overhang on all sides; spray the parchment with the cooking spray.
Make the blondie: In a medium bowl, whisk together the flour, baking powder, and salt.
In a medium saucepan, melt the butter over medium-low heat. Remove from heat and mix in the brown sugar and vanilla. One at a time, mix in the eggs; mix in the flour mixture until just combined (do not overmix).
Spread the batter into the prepared pan and bake until a toothpick inserted in the center comes out with moist crumbs clinging to it, 25 to 28 minutes. Cool completely in the pan.
Make the praline topping: In a small saucepan, heat the brown sugar and butter over medium heat, stirring occasionally, until bubbling, about 2 minutes. Remove from heat and whisk in the vanilla, 1 tablespoon of water, and confectioners’ sugar until smooth. Spread over the cooled blondies and sprinkle with the pecans. Let the topping set, 10 to 15 minutes.
Holding both sides of the paper overhang, lift the cake out of the pan, transfer to a cutting board, and cut into 16 squares. Store the blondies in an airtight container at room temperature for up to 5 days.