Preheat oven to 375°F. Lightly oil an 8-inch square baking pan or coat it with nonstick spray.
In a bowl, whisk flour, 1/2 cup sugar, baking powder, baking soda and salt.
In a large bowl, whisk egg, yogurt, oil and vanilla until smooth. Add dry ingredients and stir with a rubber spatula until just blended. (Do not overmix.) Fold in pineapple. Scrape batter into prepared pan.
In a small bowl, combine pecans and remaining 3 tablespoon sugar. Sprinkle over batter.e for 25 to 35 minutes, or until top is golden and a skewer inserted in the center comes out clean. Let cool in the pan on a wire rack for about 20 minutes. Cut into squares and serve warm.