Prepare loaf pan and oven. Spray a 9x5 inch loaf pan and line with parchment paper. Pre-heat oven to 350°F.
Mash bananas. In a large bowl, mash the bananas until smooth.
Add melted butter. Melt the butter in a bowl in the microwave and whisk into the banana.
Add sugar and wet ingredients. Whisk the sugar, egg, and vanilla extract into the banana-butter mixture until smooth.
Combine dry ingredients. In a separate bowl, sift the flour, baking soda, and salt together.
Fold together. Using a rubber spatula, fold the dry ingredients into the wet until just combined.
Make strawberry batter. Separate out 300 grams of the banana bread batter into a separate bowl. Fold in the freeze-dried strawberry powder into this bowl.
Finish banana bread batter. Fold the ground cinnamon into the original batter without the strawberry powder.
Marble and bake. Dollop the batters into the prepared loaf pan in alternating spoonfuls. Run a cake tester, or the back of a spoon, through the batters to swirl them together. Bake in a 350°F oven for 55 minutes.
Cool for 10 minutes. Once a cake tester inserted into the strawberry banana bread comes out clean, transfer the loaf pan to a wire rack and allow to cool for 10 minutes in the pan. Lift the bread out of the pan and allow to finish cooling on the rack. Banana bread will keep at room temperature for 3 days or tightly wrapped in the refrigerator for 5.