Lightly grease an 8 or 9-inch cake round and line it with a circle of parchment paper.
In a medium bowl, whisk together the 1 ¾ cup gluten free flour, 1 ½ teaspoons baking powder, and ½ teaspoon fine sea salt
In a large bowl, use an electric mixer to cream the ½ cup unsalted butter and 1 cup granulated sugar until light and fluffy, about 2 minutes. Beat in the 2 large eggs one at a time, mixing thoroughly on medium speed until well combined.
On low speed, beat in half of the flour mix, then the ½ cup whole milk, and finish with the remaining flour. Using a rubber spatula, gently fold in the 2 cups fresh or frozen blackberries.
Preheat oven to 350℉. Pour the batter into the prepared pan and smooth the top. Scatter the ½-1 cup sliced almonds across the top. Let batter the rest while oven heats, then place pan in oven and bake for about 45-50 minutes, checking at 40 minutes and at regular intervals, until the cake's surface is slightly firm, a tester inserted in the center comes out clean, and the edges are just pulling away from the pan.
Remove cake from oven, place on a wire rack, and let cool for 20 minutes in the pan, then turn out onto a rack to cool completely.
Notes
Rest the batter. After you gently mix in the flour, let the batter rest while the oven heats so that the flour can hydrate.
Use room temperature ingredients. Bring your ingredients to room temperature for about 15 minutes before using them, so they can better incorporate into the batter.
Buckwheat flour. Let's take it back to my original rendition of this cake! Buckwheat flour is a whole grain flour (if you're wondering, is buckwheat gluten free?, yes, it is!) that holds up well to chocolate and berries. Try ½ buckwheat flour and ½ gluten free all-purpose flour to add extra whole grains.
To make dairy-free - Substitute your favorite non-dairy butter for the butter and use non-dairy milk in place of the milk.
Substitute another fruit. I know, I know, this is a blackberry cake. But it's also wonderful made with blueberries or raspberries.