Line a 9-inch square baking pan with parchment paper, allowing for overhang on all sides.
For the crust and topping: Whisk together flour, 1 cup oats, brown sugar, baking powder, lemon zest, and salt in a medium bowl. Add melted butter and vanilla, stirring until the mixture resembles moist, crumbly sand.
Press ⅔ of the mixture (about 2 cups) into the prepared pan. Bake for 10 minutes and set aside to cool slightly.
For the filling: Place cornstarch in a small bowl. In a medium saucepan over medium heat, combine cherries, sugar, and lemon juice. Cook for 4-6 minutes until cherries release their juices.
Stir a couple of spoonfuls of the juice from the pan into the cornstarch and mix until dissolved. Pour the cornstarch mixture back into the saucepan and stir. Bring to a boil and cook for 10 minutes, stirring occasionally, until thickened.
Remove from heat and stir in almond extract.
Spread the cherry filling evenly over the slightly cooled crust.
Mix 2 tablespoons oats into the remaining crumble mixture. Sprinkle over the filling and lightly press it down with a spatula.
Bake for 40-50 minutes until the topping is lightly browned and a toothpick comes out mostly clean with a few cherry specks.
Allow the bars to cool completely in the pan on a wire rack. After 1 hour, refrigerate to speed up cooling.
For the optional almond icing: Whisk together confectioners' sugar, milk, and almond extract until smooth.
Drizzle the icing over the cooled bars and cut into squares.
Notes
These cherry pie bars are a delicious, easier alternative to a traditional pie. The almond icing adds a nice touch of flavor, but it's optional if you're looking for a more rustic treat.