Preheat the oven to 350°F (175°C). Grease a 9-inch square baking pan and line the bottom with parchment paper. In a medium bowl, whisk the flour, baking powder, baking soda, and salt until evenly combined.

In a large bowl, beat the softened butter and sugar until creamy and fluffy. Add the eggs one at a time, mixing well after each one. Stir in the vanilla extract.

Add the flour mixture to the butter mixture in a few additions, alternating with the buttermilk. Stir gently after each addition until the batter comes together and looks smooth. Do not overmix.

To make the crumble, mix the chopped pecans, flour, brown sugar, and cinnamon in a bowl. Add the cold butter and work it into the mixture with your fingertips until you have coarse, crumbly pieces.

Spread the batter evenly in the prepared pan. Scatter the cinnamon-pecan topping over the surface. Bake for 35–40 minutes, until the cake is set in the center and a toothpick comes out clean or with a few moist crumbs.

Leave the cake in the pan for 10–15 minutes, then move it to a wire rack to cool. Once slightly cooled, cut into 9 pieces and serve warm or at room temperature.