Grease and line the bottom of two 6-inch round cake pans with a circle of parchment, or use an 8 or 9-inch cake round.
In a small bowl, whisk together the 1 ¾ cups homemade gluten free flour blend, 1 ½ teaspoons baking powder and ⅛ teaspoon salt.
In a large bowl and using an electric mixer on medium speed, cream the ½ cup unsalted butter, scraping down the sides of the bowl as necessary, until the butter is lighter in color, about 45 seconds. Then add the 1 cup granulated sugar in a steady stream and mix on medium speed until the mixture is very light in color and texture, about 4 to 5 minutes.
Add the 2 eggs one at a time, and beat very well for about 2 minutes. Beat in the ½ cup whole milk and 1 teaspoon pure vanilla extract.
With the mixer on low, add the flour mixture in four doses, alternating with the milk in three doses. Scrape down the sides of the bowl in between additions.
Pour the batter into prepared cake pan. Preheat the oven to 350℉ and let the batter rest while it heats, up to 30 minutes. Place the cake in the oven and bake for 25 to 30 minutes, or until a toothpick inserted in the center comes out clean.
Remove from the oven and set on a rack to cool for 10 minutes then turn out onto rack to finish cooling completely.
To assemble: Whip the 1 ½ cups heavy cream until soft peaks form. Split each cake round in half, and place the bottom layer on a plate. Spread it with ⅓ of the 1 cup blackberry jam and top it with several spoonfuls of whipped cream. Repeat this until you reach the final cake layer. Place it on top, then cover and frost the sides with the remaining whipped cream.
Notes
What size pan: I love a wee cake, so I baked this recipe in two 6-inch cake rounds. This also allowed me to have four layers, important to me when making a celebratory cake. The batter itself can be baked in an 8 or 9-inch round. You can split the round in half, a la my gluten free rhubarb cream cake, then top the bottom layer with jam and whipped cream, place the top half on, and finish the top and sides with whipped cream.
Chill the cake before assembling. After the cake has cooled to room temperature, place it in the fridge for at least 30 minutes and up to overnight. This will help the crumb firm a bit, making it easier to cut the cake layers.
Refrigerate before serving. For the cleanest slices, chill the fully assembled cake for 30-60 minutes before cutting into it to allow the whipped cream to firm up a little.
Enjoy cold or at room temperature. I love nibbling on slices of cake straight from the fridge, and it's equally delicious after sitting out on the counter for 15-20 minutes at room temperature to soften the crumb a bit.
Make it dairy-free. Swap the heavy cream for coconut cream, the milk for your favorite non-dairy milk, and the butter for non-dairy butter.
Try buttercream. You could also make a vanilla or blackberry buttercream frosting instead of the cream: Beat 1 cup of unsalted, softened butter with vanilla extract or jam, then add 3-4 cups of powdered sugar.