In a small saucepan over medium-high heat, combine the red beans and sugar. Cook for 30 minutes, stirring constantly, until a thick paste forms. Remove from the heat and let cool completely.
Preheat the oven to 350F. Line and oil a 9-inch cake pan, with tabs to pull the cake out if you’d like to pull it out before consuming. Combine all of the crumb ingredients minus the butter in a medium-sized bowl. Cut up the cold butter into cubes and add to the dry ingredients. Use your fingers to squeeze the butter into the mixture until the largest mounds are pea-sized. Set the bowl in the fridge while you make the cake batter!
In a large bowl, whisk the yogurt and sugar together until fully smooth. If you’re adding sourdough starter, whisk it in now. Then add the eggs one at a time, whisking until totally combined between each one. Add the vanilla extract and whisk thoroughly.
In a small bowl, combine the flour, baking powder, baking soda, and salt. Sift the mixture over the wet cake ingredients and use a spatula to fold it in. When halfway combined, pour in the melted butter while folding. Mix until just combined.
Transfer the batter into the prepared cake pan. Using your hand, evenly break up the red bean paste over the batter so it’s covered in an even layer. Evenly sprinkle all of the crumb topping over the red bean paste layer. It’s okay if some of the crumbs are literally just crumbs!
Bake for 50 minutes, and let cool for 15 minutes in the pan. Remove from the pan if you wish before serving, or don’t! This cake tastes amazing a bit warm. Let me know what you think below in the comments or on Instagram @higheralchemybaking!