Preheat the oven to 350F. Line a 8 x 8-inch baking pan with parchment paper, leaving a couple of inches of overhang.
Add all the ingredients for the streusel together. It should form clumps on its own, but press the rest together to encourage more clumps. Set aside, preferably in the freezer.
Mix together the flour, baking powder, baking soda, and salt for the cake together in a bowl and set aside.
Beat together the butter and sugar until light and fluffy in a large mixing bowl or stand mixer. Add in the egg and beat well for at least another two minutes. Add in the bananas and beat well for an additional 3 to 4 minutes. Add in the yogurt and beat for another minute. Scrape down the bottom and sides of the bowl frequently.
Add the flour mixture into the wet ingredients and stir until just combined and there are no streaks of flour. There may be a couple of lumps, and that is okay,
Mix together the ingredients for the swirl mixture into a small bowl.
Pour half the batter into the baking pan, then top with the swirl. Add the remaining batter and top with the crumb topping. Lightly press down on the streusel to ensure it adheres.
Bake for about 35 – 40 minutes, or until the cake is golden brown and a toothpick inserted comes out clean or with a few crumbs clinging to it.
Notes
Use overripe bananas and room temperature ingredients unless otherwise noted. For the best results, weigh out your dry ingredients. The cake can be stored at room temperature in an airtight container for up to four days.