Duplicate that smaller black shape (so you have 3 black shapes in your layers palette: 1 bigger and 2 smaller ones). Change the color of the new shape to satuated brown #401c02. Hit Ctrl+Alt+G so the layer becomes a clipping mask. Then go to the Layers Palette, add a layer mask to this brown shape layer (if it isn’t already there), select the mask and hit Ctrl+I to invert the colors. This will make the brown button disappear.