In a bowl to fit - Add diced rhubarb with 1T. flour - toss to coat; set aside.
Sift 2 1/2 cups flour, baking soda, sea salt, cardamom and ginger into a medium sized bowl; set aside.
Preheat the oven to 350 degrees (I used convection-bake.) Add a piece of parchment to 9 " x 13" baking pan, add apiece of parchment to the bottom of the pan, grease with butter all around.
Cut butter into tablespoons, place in a microwaveable glass dish and microwave until just melted (or heat over the stove in a small skillet.)
Add the melted butter to a large bowl and let stand 5 minutes (to cool off.) Add the sugar - use a wire whisk to combine - whisk for 30 seconds until light and creamy. Add eggs, one at a time whisking in between; whisk all now for one minute until very creamy. Add vanilla and blend in.
Add 1/3 of the flour mixture; whisk in. Add 1/2 cup buttermilk: whisk in. Continue adding 1/3 flour - whisk in, then the remaining 1/2 cup buttermilk - whisk in, and the final 1/3 of flour mixture - USING a rubber spatula now to combine all in large strokes until just blended.
Add the diced rhubarb and fold in evenly. Add the batter to the prepared pan, smoothing the top. Sprinkle 1 tablespoon of sugar evenly on the top and bake 25 minutes until nicely risen, golden, and springy to the touch. See below **