Shred zucchini on the large holes of a box grater, place in a clean dish towel, and squeeze out excess moisture until only slightly damp; preheat oven to 350°F.

Spread pecans on a rimmed baking sheet and toast in the oven for 6–8 minutes until fragrant; cool and chop coarsely.

Line a 9×13-inch pan with parchment and whisk together flour, baking soda, baking powder, salt, and cinnamon in a bowl.

In a separate large bowl whisk oil with brown sugar and granulated sugar until combined, add eggs, vanilla, and yogurt, then fold in the dry ingredients just until combined.

Fold in the drained zucchini and 1 cup chopped toasted pecans, then spread batter evenly into the prepared pan.

Bake at 350°F for 30–36 minutes, rotating once; a toothpick should come out with a few moist crumbs. Cool in the pan 20–30 minutes, then lift out with parchment and cool completely on a wire rack.

Melt butter in a small saucepan over medium heat, stir in brown sugar and heavy cream, and simmer gently for 2–3 minutes while whisking; remove from heat and whisk in powdered sugar and vanilla until smooth and spreadable.

Chill the frosting briefly if it’s thin, then spread over completely cooled bars, sprinkle with remaining pecans if using, and chill 15–20 minutes to set before slicing into 12–16 squares.