Preheat your oven to 350°F / 175°C and grease a 24-cup muffin tin (or two 12-cup tins). In a medium bowl, whisk together the flour, baking powder, baking soda, and salt, then set aside. Finely shred the yellow squash using a box grater or food processor, wrap it in a clean kitchen towel, and squeeze firmly to remove as much moisture as possible — this step is essential to prevent soggy muffins. Make sure your eggs are at room temperature before you begin.

In a large mixing bowl, whisk together the sugar, oil, lemon extract, and room-temperature eggs until the mixture is pale and well combined, about 1 to 2 minutes. Stir in the lemon juice and lemon zest, then gently fold in the sour cream until just incorporated.

Add the squeezed-dry shredded squash to the wet ingredient bowl and fold it in until evenly distributed. Pour the dry ingredient mixture over the wet ingredients and fold together with a spatula until just combined — stop as soon as the flour disappears. Do not overmix; the batter should look thick and slightly lumpy.

Divide the batter evenly among the prepared muffin cups, filling each about two-thirds full. Bake at 350°F / 175°C for 22 to 25 minutes, until a toothpick inserted into the center of a muffin comes out with just a few moist crumbs. The tops should be golden brown and slightly domed.

While the muffins bake, make the glaze: sift the powdered sugar into a small bowl, then whisk in the vanilla extract, freshly squeezed lemon juice, and a pinch of fine salt until smooth and pourable. The glaze should be thick enough to coat a spoon but thin enough to drizzle.

Remove the muffins from the oven and let them rest in the pan for 2 to 3 minutes, then transfer to a wire rack. While the muffins are still warm (but not piping hot), drizzle or spoon the lemon glaze over the tops, letting it run down the sides slightly. Allow the muffins to cool completely on the rack so the glaze sets and the flavors come together before serving.