Lightly grease an 8 x 8-inch metal baking pan. (A metal pan is strongly recommended for the best texture and even baking. Avoid using glass.) Line the pan with a strip of parchment paper, leaving an overhang on two sides to make removing the cake easy after baking. Alternatively, you may use a 9-inch round springform pan.

In a medium bowl, whisk together the gluten-free flour, baking powder, and salt. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, or using a handheld electric mixer, beat the softened butter and granulated sugar on medium speed for about **2 minutes**, until well combined, slightly lighter in color, and a little fluffy.

Add the lemon zest, eggs, vanilla extract, and milk. (For the smoothest batter, make sure all of your refrigerated ingredients are at room temperature.) Mix until combined.

Add the dry ingredients mixing just until incorporated. The batter will be fairly thick.

Let the batter rest for 30 minutes. This allows the gluten-free flour to fully hydrate and results in a better texture.

While the batter is resting, preheat the oven to 350°F (175°C).

Make the crumb topping: In a small bowl, combine the brown sugar, flour, and melted butter with a fork until evenly moistened and crumbly. Set aside.

After the batter has rested, gently fold in the fresh blueberries.

Spread about half of the batter into the prepared pan, smoothing it into an even layer.

Sprinkle half of the crumb mixture evenly over the batter.

Dollop the remaining batter over the crumb layer in small spoonfuls across the surface. Once it is distributed, gently spread and connect the dollops into an even layer. This helps cover the crumb layer without dragging it through the batter.

Sprinkle the remaining crumb topping evenly over the top.

Bake for 45–55 minutes, depending on the pan you are using. A 9-inch round springform pan will typically bake closer to 45 minutes, while an 8 x 8-inch square pan generally takes 50–55 minutes because the cake is slightly thicker. The cake is done when the top is golden brown and a toothpick inserted into the center comes out clean or with just a few moist crumbs.

Allow the cake to cool completely in the pan.

Make the glaze: In a small bowl, whisk together the powdered sugar, fresh lemon juice, and heavy cream (or milk) until smooth. Adjust the consistency with a little more lemon juice or powdered sugar if needed.

Once the cake has cooled completely, drizzle the glaze over the top. Slice and serve.