Preheat oven to 325°F (163°C). Line a 9x13-inch pan with parchment paper (leave overhang for lifting).
In a mixing bowl, beat the softened butter and powdered sugar until smooth and creamy (about 1–2 minutes).
Add the flour and mix on low just until a crumbly dough forms.
Press the dough together gently until it holds (do not overwork).
Press the dough evenly into the lined pan and smooth the top.
Prick the dough all over with a fork (docking). Lightly score into 24 rectangles for easy cutting later.
Bake for 28–35 minutes, until the edges are lightly golden and the center looks set.
Cool in the pan for 15–20 minutes, then lift out using parchment. Cut fully along the scored lines. Cool completely before storing.