Preheat oven to 350 degrees F. Line a large baking sheet with parchment paper and set aside.

In a large bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.

In another large bowl, using an electric hand mixer on medium speed, blend the sugar, egg, vegetable oil, vanilla extract, and milk until well combined.

Add the dry ingredients to the wet ingredients, stirring with a wooden spoon until all dry ingredients are incorporated.

Stir the rhubarb pieces into the cookie dough until combined.

Scoop ¼ cup portions of cookie dough onto prepared baking sheet. Flatten cookies with the palm of your hand or a wide bottom glass jar.

Bake for 10-12 minutes, until cookies are no longer wet on top and bounce back slightly when pressed in the center of the cookie. Remove cookies from baking sheet and cool on a wire wrack.

While the cookies are baking, mix together the confectioners sugar and lemon juice. Dip the tops of cookies in the glaze, then return to the baking sheet, allowing the glaze to dry completely before storing.