How to Make It
Preheat the oven to 350°. Coat a 9-inch-square cake pan with vegetable oil spray.
In a food processor, pulse the brown sugar with the flour, cinnamon and salt. Add the butter and pulse until the mixture has a sandy texture. Add the oats and pecans and pulse until incorporated. Transfer the crumbs to a bowl and freeze until ready to use.
In a medium bowl, combine the flour with the baking powder, baking soda, cinnamon and nutmeg. Sift the mixture onto a sheet of parchment paper.
In a large bowl, using an electric mixer, cream the butter with the brown sugar and salt at medium-high speed until fluffy, about 5 minutes; scrape the bowl. Add the sour cream and beat until smooth, about 2 minutes. Beat in the eggs one at a time at medium speed, scraping the bowl after each addition. Beat in the vanilla. At low speed, beat in the flour mixture in 3 additions, scraping the bowl as needed.
Spread the batter in the prepared pan and scatter the frozen crumbs on top. Bake for 30 minutes, until golden brown and a toothpick inserted in the center comes out clean. Transfer to a wire rack and let cool. Cut into 12 squares and serve warm or at room temperature.