Preheat oven to 350ºF. Line a 9 inch round or 8 inch square metal baking pan with parchment and set aside. (If using a round pan, spray pan with cooking spray before placing parchment into the pan, since parchment will not cover sides of pan.)
Make the blueberry cake.
Place blueberries in a small bowl and toss with 2 Tablespoons flour until all berries are dusted with flour. Set aside.
In a medium bowl combine remaining 1 ¼ cups flour, baking powder, and salt. Whisk to combine; set aside.
In the bowl of a stand mixer (or in a large bowl using a handheld electric mixer), cream butter and sugar. Add egg and beat well to combine.
With the mixer running on low speed, add flour mixture and stir just until combined. Then add the milk, stirring on medium speed just until blended. Add the floured blueberries and fold them into the batter using a spatula or wooden spoon.
Pour the batter into the prepared baking pan and use a spatula or spoon to spread it gently into a smooth, even layer. Bake for 45-50 minutes or until cake is golden and a toothpick inserted into the center of the cake comes out clean. Let the cake cool in the pan on a wire rack before removing from the pan.
Make lemon sauce.
In a medium saucepan, add sugar, cornstarch, and salt. Whisk to combine and set aside.
Boil water and add to the saucepan with the sugar.
Cook the mixture over medium-high heat, stirring constantly, until it comes to a gentle boil and begins to thicken and become translucent, about 5 minutes. The mixture should be thick enough to coat the back of a spoon.
Add the lemon zest and juice to the pan and whisk to blend. Add the butter to the pan, 1 cube at a time, and whisk until melted.
Continue to cook the mixture for about 5 more minutes, stirring constantly. The sauce will come to a rolling boil and is ready when it's thick enough to coat the back of a spoon.
Allow the sauce to cool for at least 20 minutes before pouring over the cake. The sauce will continue to thicken as it cools, and it should be warm but not hot when poured over the cake.
Slice the cake and serve warm or at room temperature. Garnish with lemon slices and fresh blueberries, if desired.
Notes
Storage: Store leftover cake in an airtight container at room temperature for up to 3 days.