Preheat oven to 325 degrees. Cream butter and sugars until smooth. Add pecans, oats, flour, baking powder, cinnamon, and a pinch of salt. Stir until mixture begins to form large clumps. Transfer mixture to a rimmed baking sheet, and spread in a single layer. (Clumps should be close together.)
Bake until golden brown, about 25 minutes. Let cool completely in sheet on a wire rack. Using your hands, break into small pieces.
Crumble can be stored for up to 1 week.