Preheat oven to 350 degrees and line a 9 x 9 square pan with aluminum foil or parchment paper.
In a large bowl, cream butter and sugar together until smooth and light colored.
Add egg, vanilla extract, baking powder, and sea salt and stir to combine.
Add flour, half at a time, and mix until just incorporated.
Using the back side of a spoon floured well, gently press the cookie down into a lined baking pan. Continue flattening until it is even and all the way across the bottom of the pan.
Bake for 20 minutes, rotating halfway through. The edges will be slightly brown around the edges and the center may puff up slightly. This will settle when cooling.
Remove from oven and allow cookie bars to cool in the pan for 10 minutes before lifting out to cool completely.