• Preheat your oven to 350°F. Grab an 8x8-inch square pan and give it a light spray with cooking spray, or line it with parchment paper. Set it aside for now.

Make the streusel topping:

• In a medium bowl, stir together the flour, sugar, and cinnamon. Pour in the melted butter and use your hands to mix until it looks crumbly and delicious. Set aside.

Now for the cake:

• In another medium bowl, whisk the flour, baking soda, baking powder, and salt. That’s your dry mix, set it aside.
• In a separate large bowl, beat together the butter and sugar with an electric mixer on medium speed for about 2 minutes, until it’s nice and fluffy. Don’t forget to scrape the sides of the bowl. Add the egg, sour cream, and vanilla, and beat for another minute until everything is smooth.
• Switch the mixer to low speed and slowly add in the dry mix. Blend until just combined, don’t overmix! Scrape down the sides as you go to keep things even.
• Pour the batter into your prepared pan and spread it out with a spatula. It will look thin, but trust me, it puffs up beautifully in the oven. Sprinkle the streusel all over the top and gently press it down with your fingertips. It might feel like too much topping, but honestly, that’s the best part!
• Bake for 30 to 35 minutes, until the cake is set and a toothpick poked into the center comes out clean.
• Take the cake out and place the pan on a wire rack to cool for about 15 minutes before slicing.