Line the pan with a piece of parchment paper, cut so that it extends on two sides to easily remove it from the pan.
Grease the exposed parts of the pan with butter or oil to prevent sticking.
In a small bowl, whisk together the flour, baking powder, baking soda, and kosher salt.
Place the lemon zest in a large bowl with the sugar. Use your fingers to rub the zest into the sugar until it is fragrant (this activates the flavor of the zest).
Whisk in the melted butter until smooth.
Whisk in the eggs one at a time, then the lemon extract. Switch to a spatula.
Add the dry ingredients and buttermilk and gently stir until smooth.
Spread the batter evenly into the prepared pan.
Bake 30 to 35 minutes or until top is light golden brown and a toothpick inserted in center comes out clean.
Cool 20 minutes in the pan. Use the parchment paper to remove the cake from the pan and allow to cool to room temperature.
Meanwhile, mix together the powdered sugar with the lemon juice, adding slightly more if necessary to get to a thick but pourable consistency.
Pour the glaze over the cake and smooth with a spatula.
Cut into pieces with a serrated knife within the first few minutes, before the glaze dries. If desired, top with more lemon zest or lemon slices.
Store in a sealed container at room temperature for up to 3 days or refrigerated for 1 week. (Bring to room temperature before serving.)