01 - Preheat oven to 350°F. Grease and flour an 8-inch square baking pan or line it with parchment paper.
02 - Combine flour, brown sugar, cinnamon, and salt in a bowl. Cut in cold butter using a fork or fingertips until mixture forms coarse crumbs. Refrigerate until ready to use.
03 - In a medium bowl, whisk together flour, baking powder, baking soda, and salt.
04 - Beat softened butter and granulated sugar in a large bowl with an electric mixer until light and fluffy, about 2 minutes.
05 - Add eggs to the butter mixture one at a time, beating well after each. Mix in vanilla extract.
06 - Combine cinnamon soda and sour cream in a separate bowl or measuring cup.
07 - Add dry ingredients to butter mixture in three additions, alternating with the soda and sour cream mixture, beginning and ending with dry ingredients. Mix until just combined.
08 - Gently fold rainbow sprinkles into batter.
09 - Spread batter evenly into prepared pan. Sprinkle cinnamon streusel over the top.
10 - Bake for 32 to 37 minutes, until a toothpick inserted in the center emerges mostly clean.
11 - Allow to cool in the pan for 15 minutes, then remove and let cool completely before slicing.