Arrange a rack in center of oven; preheat to 350°. Grease a 9" cake pan with cooking spray.
In a small saucepan over medium heat, cook butter, brown sugar, and syrup, stirring frequently, until butter is melted and mixture is smooth, about 5 minutes. Remove from heat and stir in pecans.
Pour into prepared pan. Refrigerate at least 20 minutes and up to 1 day.
In a medium bowl, whisk flour, cinnamon, salt, and baking powder.
In a large bowl, using a handheld mixer on medium-high speed (or in the large bowl of a stand mixer fitted with the whisk attachment), beat granulated sugar, butter, and brown sugar until light and fluffy, 2 to 3 minutes. Add eggs, one at a time, beating to blend after each addition. Beat in vanilla.
Add dry ingredients and beat on medium-low speed until just a few dry streaks remain. Add milk and beat on medium speed until just combined. Gently pour batter over pecan topping; smooth in an even layer.
Bake cake until a tester inserted into the center comes out clean, 40 to 45 minutes. Let cool 10 minutes, then invert onto a cutting board or platter. Let cool completely.