Preheat the oven to 350 degrees.
Add the 8 Tbsp softened butter, and 1 cup brown sugar in a large bowl or a stand mixer. Use a handheld mixer if you don't have a stand mixer and beat the butter and sugar together for 2 minutes or until combined and fluffy.
Add eggs and vanilla and mix until just combined.
Add the 1¼ cups all purpose flour, 1 tsp baking powder, ¾ tsp salt by mixing them in with a rubber spatula to as not to over-mix the batter.
Mix in the sourdough discard with the spatula until combined.
Add the mashed bananas and gently fold into the batter so as not to over-mix.
Cut out squares of parchment paper to use as muffin papers, or use regular muffin papers in the muffin tins. Make sure they are taller than muffin insert so they can hold the batter plus the streusel.
Fill 7 muffin tins with liners and scoop ⅔ full. Add equal amounts of the crumble to the tops of each muffin. If you have a jumbo muffin tin tray, then make 6 muffins.
Bake at 350 degrees in the middle oven rack for 28-30 minutes or until a toothpick inserted in the center comes out clean.
Let cool for a few minutes before serving.