Place a piece of parchment in an 8-inch cake round. Grease the parchment and sides of the cake pan with butter.
In a large bowl, using an electric hand mixer on medium speed, cream the ½ cup unsalted butter and 1 cup granulated sugar until light and fluffy, about 3 minutes. Add the 2 eggs one at a time, beating until combined after each and scraping down the sides and bottom of the bowl as needed.
In a medium bowl, whisk together the 1 ¾ cups gluten free flour, 1 ½ teaspoon baking powder, and ½ teaspoon fine sea salt.
With the mixer on low, stir half of the flour mixture into the butter mixture until just combined. Stir in the ½ cup whole milk until just combined, then add the rest of the flour, scraping down the sides and bottom of the bowl as needed.
Position a rack in the center of the oven and heat to 350º F. Spread the batter evenly in the pan and let rest while the oven heats.
Place the cake in the oven and bake until the top is golden and a toothpick inserted near the center comes out clean, the top is set, and the edges are slightly coming away from the pan, 40-50 minutes. Remove from oven and let cake cool completely in the pan on a rack, then invert onto a serving platter and peel away the parchment.
Using a serrated knife, split cake in half horizontally; place bottom half, cut side up, on a serving plate.
Assembly
In a large bowl, whip the 1 ½ cups heavy cream until soft peaks form.
Spread the bottom cake layer with the 1 cup rhubarb compote. Spread half the whipped cream over and top with the other cake layer. Use the remaining whipping cream to frost the top and sides of the cake.
Cake may be kept, well covered in the fridge, for 3 days.
Notes
CN's Recipe Notes
Sweeten the whipped cream. Note that I don't call for sweetening the whipped cream before putting the cake together. However, for a sweeter whipped cream, whip in ¼ cup of sifted powdered sugar.
Chill the cake before assembling. After the cake has cooled to room temperature, place it in the fridge for about 30 minutes. This will help the crumb firm a bit, making it easier to cut the cake in half.
Refrigerate before serving. This is a helpful, though not 100% necessary step. For the cleanest slices, chill the fully assembled cake for 30–60 minutes before cutting into it.
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.
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.