Preheat the oven to 375 degrees F. Line a 9x9-inch square baking pan with parchment paper or spray lightly with nonstick cooking spray. Set aside.
Make The Streusel : In a medium bowl, whisk together the flour, brown sugar, and ground cinnamon until combined. Add in the sliced butter and use clean hands, a fork, or a pastry cutter to cut in the butter, until a crumbly mixture forms.
2/3 cup all-purpose flour, 1/2 cup packed light brown sugar, 1½ teaspoons ground cinnamon, 3 tablespoons salted butter
Cover the bowl with plastic wrap, or a lid, and place it in the fridge while you prepare the cake batter.
Make The Blueberry Buckle Cake Batter : In a large bowl, whisk together the flour, granulated sugar, and baking powder until combined.
2 cups all-purpose flour, 1 cup granulated sugar, 2 teaspoons baking powder
Add in the egg, milk, melted butter, and vanilla extract. Use a silicone mixing spatula to gently mix everything together until a lumpy batter forms and no flour pockets remain in the batter.
1 large egg, 2/3 cup whole milk, 1/2 cup (1 stick) salted butter, 1 teaspoon vanilla extract
Gently mix in the frozen blueberries, making sure to not overmix and bleed the blueberry color into the batter.
1 cup frozen blueberries
Evenly spread the cake batter into the prepared pan. Remove the streusel from the fridge and evenly sprinkle it over top the cake batter.
Bake : Bake for 35-40 minutes or until the streusel is golden brown. You may want to loosely lay a piece of aluminum foil over the cake halfway through the bake time so the edges don't burn.
Remove the cake from the oven and place it on a cooling rack. Allow the cake to cool for 10-15 minutes inside the pan, and then slice and serve the blueberry buckle cake warm or you can let it cool completely and serve it at room temperature.