2/3 cup sugar, white or brown
1 1/2 Tbsp flour
1/4 tsp cinnamon or nutmeg
1 Tbsp softened butter
3 cups sliced rhubarb, 1/4 inch pieces
1 1/2 cups flour
1 1/2 tsp baking powder
1/2 tsp baking soda
1/2 tsp salt
1/4 tsp cinnamon, nutmeg or cardamom (optional)
2 eggs, separated
1/3 cup butter, room temp
3/4 cup sugar
1 tsp vanilla extract
3/4 cup plus 2 Tbsp buttermilk or orange juice
2 tsp grated lemon zest (optional)
Whisk the sugar, flour and cinnamon/nutmeg together. Work the butter in with your fingers until it resembles cornmeal. Toss the rhubarb with this mixture and set aside.
Whisk the flour with the baking powder, baking soda, salt and spices, if using. Add any or all three of the spices or if you want a plain vanilla cake, omit the spices. Separate the egg and whip the whites with 2 Tbsp of the sugar until stiff peaks form and set aside.
Beat the butter and sugar until light and fluffy. Add the egg yolks and vanilla and beat well.
By hand, or using low speed, add about a third of the dry ingredients. Before it is completely smooth, add half the buttermilk or orange juice. When it's almost incorporated, add another third of the dry ingredients. Then add the rest of the buttermilk or orange juice. Finish with the remaining dry ingredients. Do NOT overbeat if you want a tender cake.
Fold in half the egg whites by hand to loosen the batter, then fold the second half in.
Spray a bundt pan or a 6 or 7-inch pan (cheesecake pans work fine for this recipe) with baking spray.
Spread about half the rhubarb/sugar/flour mixture in the bottom of the pan and pour half the batter over it. Add the rest of the rhubarb and cover with the remaining batter. No need to cover the pan.
Put 1 1/2 cups of water in the Instant Pot. Put the cake on the trivet and close the pot. Set the valve to Sealing. Push Pressure Cook (or Manual) and adjust time using the + or - button to get to 60 minutes. When it beeps that it's done, leave it for 15 minutes natural release. Flip the valve to Venting and when the pin drops, open the pot.
Let the cake cool on the counter. When ready to unmold, fold a tea towel in four and run it under hot tap water. Wring it out and set the bottom of the pan on it for 30 seconds or so. This will help the rhubarb layer to release from the pan. Invert the cake onto a plate and slowly remove the pan.
Serve with sweetened whipped cream or ice cream.