Preheat your oven to 350°F and line a baking sheet with parchment paper. While the oven heats, dice the rhubarb into small 1/4-inch pieces and place in a bowl—this prevents it from being distributed unevenly in the dough. Zest the lemon and squeeze the juice fresh; I always use fresh lemon juice rather than bottled because it has much better acidity and brightens the cookie flavor significantly. Set all prepared ingredients within arm's reach of your mixing area.

In a large mixing bowl, combine the softened vegan butter, packed brown sugar, and sugar. Beat these together until the mixture is light, fluffy, and pale—this usually takes about 2-3 minutes with an electric mixer or vigorous hand mixing. This creaming process incorporates air into the dough, which helps the cookies rise and achieve a tender crumb. The vegan butter I use from Miyoko's Creamery whips beautifully and creates a genuinely buttery texture in the final cookie.

In a separate bowl, whisk together the flour, baking powder, and salt until evenly combined. This whisking distributes the leavening agent and salt throughout the flour, ensuring they'll be evenly incorporated into the dough rather than creating pockets of concentrated baking powder flavor.

Add the dry ingredient mixture from Step 3 to the creamed wet ingredients from Step 2, stirring until just combined—don't overmix, as this can develop gluten and create tough cookies. Pour in the egg replacer and almond milk, mixing until the dough comes together. Fold in the lemon zest, lemon juice, and diced rhubarb from Step 1, distributing the fruit evenly throughout the dough. I like to fold the rhubarb in gently near the end so it stays intact and creates those lovely tart pockets in each cookie.

Using a spoon or small cookie scoop, drop rounded spoonfuls of dough onto the prepared baking sheet, spacing them about 2 inches apart to allow for slight spreading. Bake in the preheated 350°F oven for 15 minutes, until the edges are lightly golden and the centers are just set—the cookies will continue to cook slightly as they cool on the pan. Remove from the oven and let the cookies rest on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.