makes one 8 by 8-inch pan; adapted from Stella Moss's Rhubarb Streusel Cake for Conservation Grains
Preheat oven to 375°F. Grease 8 by 8-inch metal cake pan with melted butter, baking spray, or use parchment.
Cream butter and brown sugar using the paddle attachment in an electric mixer. Mix for a few minutes until color and texture is light.
Add eggs one at a time, mixing until fully incorporated before adding the next.
In a bowl, whisk together flours, baking powder, nutmeg or lemon zest, and salt. Add to the cake batter alternating with the buttermilk in 4 additions mixing until combined. Spread into prepared pan.
If using compote, spread over top of batter then scatter chopped rhubarb (or other fruit) over top.
Mix together in a bowl the flours, sugar, nutmeg, and salt. Drizzle over the melted butter and mix with a spatula until combined.
Crumble over the fruit layer then bake for 45 minutes to 1 hour depending on the moisture content of the fruit used. When approaching done, cake will be fragrant. When done, edges and topping will look dry vs. wet, color will be browning, and internal temperature will reach 200°F.