Preheat oven to 350 degrees F. Grease and flour a bundt cake pan. Set aside.
In the bowl of a stand mixer, or a large mixing bowl, beat together the softened butter, sugar and lemon zest until light and fluffy, about 3-5 minutes. Scrape the sides of the bowl with a spatula and then add the eggs, one at a time, until fully incorporated. Mix in the vanilla.
In a large bowl, whisk together 2 1/2 cups of flour, baking powder and salt. Add 1/3 of the flour mixture into the egg mixture along with 1/3 of the needed buttermilk and mix on low speed. Alternate with flour and buttermilk until both are fully incorporated.
In a medium bow, or large ziplock bag, toss the rhubarb with the remaining two tablespoons of flour until the rhubarb is well coated. Gently fold rhubarb into the batter with a rubber spatula until it’s distributed evenly.
Turn out cake batter into the prepared pan, using a spatula to gently spread and level it out.
Bake for about 60-70 minutes, or until the cake springs back gently when touched and a toothpick inserted into it comes out clean.
Allow cake to sit and cool in the pan on a wire rack for 30 minutes.
Meanwhile, make the glaze by whisking together all the glaze ingredients until smooth. Thin with extra lemon juice or thicken with more powdered sugar as you see fit.
When the cake is cooled, tip it out onto a serving plate and generously drizzle cake with the lemon glaze.