Heat the oven to 350°F (175°C), conventional setting. Grease a solid 9-inch round cake pan at least 2 inches deep. Line the base with parchment and grease the parchment lightly. Set the pan on a rimmed baking sheet.

Melt the topping butter in a small saucepan over medium-low heat. Stir in the brown sugar, maple syrup, cream, and 1/4 teaspoon salt. Stir until evenly combined and gently bubbling, then cook for about 1 minute. Remove from the heat. Spread the mixture over the lined pan base and distribute the pecans evenly over it.

Whisk the flour, baking powder, and 1/2 teaspoon salt together in a bowl. In a separate small bowl, stir the sour cream and milk until smooth.

Beat the softened cake butter with the granulated sugar for 2–3 minutes until lighter in color and fluffy. Beat in the eggs individually, scraping the bowl between additions, then mix in the vanilla.

On low speed, add the flour mixture in three portions, alternating with the sour cream mixture in two portions. Start and finish with flour. Stop mixing as soon as the dry streaks disappear; finish with a spatula if needed.

Spoon the batter gently over the pecans and smooth the surface without disturbing the topping. Bake for approximately 35–45 minutes, checking from 35 minutes. The center should spring back lightly, and a skewer inserted into the cake layer should have no wet batter. Loosely cover with foil if the top colors before the center is baked.

Let the pan stand on a cooling rack for 10 minutes. Run a thin knife around the edge. Place a heatproof serving plate with a rim over the pan. Using oven mitts and holding plate and pan firmly together, turn them over in one controlled movement. Lift off the pan and peel away the parchment. Hot topping can drip; keep hands clear of the edge.

Replace any pecans left on the parchment while the topping is still warm. Cool for at least 20 more minutes before cutting into 10 slices. The listed cooking time is an estimate; use the doneness cues rather than the clock alone.