Preheat the oven to 350 degrees. Spray an 8-inch square baking pain with vegetable oil spray.

In a medium bowl, whisk together the flour and baking powder and set aside.

In the bowl of an electric mixer with the paddle attachment, beat the butter and the brown sugar at a medium-high speed for 3 to 5 minutes, until light and fluffy, stopping the mixer to scrape the bowl occasionally. Add the eggs one at a time, beating after each one to incorporate. Add the vanilla extract.

With the mixer on low speed, add the dry ingredients and mix until mostly incorporated. Add the water and mix until combined.

Spoon the batter into the baking pan and smooth the surface. Bake for 25-30 minutes, or until set and a toothpick inserted in the center comes out dry. Set the pan on a wire cooling rack. When completely cool, cut into squares.