Preheat the oven to 350°F.
Grease an 8x8 square cake pan with butter and line it with parchment, leaving an overhang of parchment on a couple sides to help lift the cake out later.
Mix the dry ingredients in a bowl (flour, baking soda, baking powder, salt, cinnamon) and set aside.
1 ¾ cup all-purpose flour, 1 tsp baking powder, ½ tsp baking soda, 1 tsp cinnamon, ½ tsp sea salt
Add the melted butter and brown sugar to another large bowl and mix for about a minute using a whisk or handheld electric beater. Mix in the yogurt and peanut butter, then the mashed bananas, eggs, and vanilla and mix thoroughly.
¼ cup butter, ¾ cup brown sugar, ¼ cup Greek yogurt or sour cream, ½ cup creamy peanut butter, 3 large bananas, 2 eggs, 1 tsp vanilla extract
Add the dry ingredients and mix them with the wet until combined. Be careful not to overmix or the cake will become gummy.
Pour half of the cake batter into the prepared square pan. Spread the jam into a layer on top of the cake batter, then top the jam with the rest of the cake batter and even it out. Tip: if your jam is too thick to spread, you can warm it slightly in a pot or microwave to thin it out.
⅔ cup jam