Preheat oven to 180 degrees. Butter a 10-cup bundt pan and set aside.
In bowl, sift together 2 1/2 cups flour, baking powder, ground ginger and salt. Set aside.
In a mixing bowl and using a paddle attachment, cream together butter, sugar and lemon zest at medium-high speed for 3 to 5 minutes or until light and fluffy. Add eggs, 1 at a time, scraping down bowl after each addition. Stir in lemon extract.
Stir in the flour mixture in 3 additions alternating with buttermilk in 2 additions, beginning and ending with flour mixture and scraping down bowl occasionally. At this stage the batter will be very thick.
Then toss the rhubarb with 2 tablespoons of the plain flour, and fold half of your rhubarb into batter.
Pour roughly half of your batter into the prepared pan, sprinkle the remaining rhubarb on top, then top with remaining batter, trying to make sure the rhubarb is completely covered over.
Bake for 60 minutes or until the top of cake is firm and center springs back when lightly touched (I checked after 50 mins, then gave another 10 mins).
Cool the cake in the pan on wire rack for 10-15 minutes before inverting and removing from pan.
Meanwhile, in a bowl, whisk together sieved icing sugar, lemon juice and butter. (Note: Mixture should be thick. Add additional 1 or 2 tablespoons of icing sugar, if necessary.) Spread glaze over cake as soon as you remove cake from pan.
The finished cake is very moist, and keeps well for 3-4 days well wrapped.
The taste is not overpoweringly of rhubarb, despite the amount going in, and the lemon complements it so well. Some people may find the glaze makes it too sweet, in which case just dust with icing sugar just before serving.
I also added some red food colouring to my glaze so the pink colour would pick up the colour of the rhubarb.