Combine all of the streusel ingredients together in a bowl.
Use a pastry cutter to cut the butter into the dry ingredients until the mixture is crumbly.
Blueberry buckle:
Using an electric mixer, cream the butter and sugar on medium until light, fluffy and no longer clinging to the sides of the bowl (2-3 minutes).
Reduce the mixer to low and add the egg. Beat until just combined.
Stir together the dry ingredients (flour, baking powder, cinnamon and salt) in one bowl and the remaining wet ingredients (milk and vanilla extract) in another.
With the mixer on low, add the dry ingredients in two equal parts, alternating with the wet ingredients in one part. Mix between each new addition until just combined.
Fold he blueberries into the batter.
Pour the batter into a parchment paper lined 8"x8" cake pan. Top with struesel.
Bake at 375F for 35-40 minutes or until a toothpick inserted in the center of the cake comes out with only baked crumbs.