Image credit to: By Rebecca B. for Boston Organics
Preheat oven to 350. Butter a 10-cup bundt pan.
To make the cake, sift the 2 1/2 cups flour, the baking powder and salt together in a bowl. Using a handheld mixer with beaters or a stand mixer with the paddle attachment, cream the butter, sugar and lemon zest together on medium-high speed for 3 to 5 minutes, until light and fluffy.
Add the eggs one at a time, scraping down the sides of the bowl after each addition, then stir in the lemon oil. Stir in the flour mixture in three additions alternating with the buttermilk in two additions, beginning and ending with the flour mixture and scraping down the sides of the bowl occasionally. The batter will be very thick.
Toss the rhubarb with the 2 tablespoons of flour and fold half into the batter. Pour the batter into the prepared pan and sprinkle the remaining rhubarb on top.
Bake for 30 minutes, then rotate the pan and cook for an additional 30 minutes, or until the center springs back when lightly touched. Cool the cake in its pan on the wire rack for 30 minutes before inverting and removing the pan.
To make the lemon glaze, whisk the confectioners sugar, lemon juice and butter together. The mixture should be thick. If it's not, whisk in another tablespoon or two of the sugar. Spread the glaze over the cake as soon as you remove it from the pan.