Preheat the oven to 375 degrees and prepare a 9x13 pan by greasing the bottom and sides with butter, or a non-stick spray. Then set aside.
In a bowl add the flour, baking powder, and salt, then whisk to combine. Set aside.
In the bowl of a stand mixer, or using electric hand mixer, add the granulated sugar and the butter. Mix on high speed for about 2 minutes, until the mixture has lightened in colour and is fluffy.
Next add the eggs, vanilla, and lemon zest to the bowl of the stand mixer and mix on high speed until everything is well combined.
Now gradually add some of the flour mixture, then beat on medium low until just combined. Then add some of the milk, again beating on medium low until just combined. Repeat this alternating process until all the flour and milk has been added into the cake batter, just do not over mix.
Finally add the cleaned blueberries to the cake batter then use a rubber spatula to gently fold the blueberries evenly throughout the cake.
Pour the cake batter into the preapred 9x13 pan, and spread into an even layer. Finally get the streusel that has been chilling and sprinkle it evenly over the top of the cake.
Place the cake into the preheated oven to bake for 45-50 minutes, until the edges are golden brown. Remove from the oven and place onto a wired rack to cool, about 20 minutes, before slicing and serving.