Preheat oven to 350 degrees F (175 degrees C).

Line a muffin tin with paper liners and coat liners with cooking spray.

Combine brown sugar, pecans, and flour in a large bowl; make a well in the center.

Beat eggs in a large bowl with an electric mixer until foamy, 3 to 5 minutes. Fold in butter and vanilla. Pour into the center of the brown sugar mixture. Stir until batter is well mixed.

Spoon batter into the muffin tin, filling each cup 2/3 of the way.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, 20 to 25 minutes.