In a medium bowl, whisk together the flour, almond flour, baking powder, baking soda, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, or a hand mixer, cream together the butter and sugar on medium-high speed until pale and fluffy, 2-3 minutes.
Add the eggs one at a time, beating on medium speed until fully incorporated after each addition. Add the vanilla and almond extracts and beat briefly to combine.
Add a third of the flour mixture and beat on low speed until combined, then add half the buttermilk and beat on low speed again. Repeat with remaining third of flour mixture, the rest of the buttermilk, and end with the last third of the flour mixture. Beat on low speed until the batter is smooth, but do not overmix.
Scrape batter into the prepared springform pan and smooth it out with a spatula. Arrange the sliced rhubarb over the top in any pattern you desire - I liked a chevron pattern. Brush the rhubarb evenly with the melted butter, then sprinkle with remaining 1 tbsp sugar.
Bake for 45-50 minutes, or until edges of cake are browned and a toothpick inserted in the center of the cake comes out clean. Cool completely before serving.
*if you don't have buttermilk, you can substitute an equal amount of sour cream or full-fat Greek yogurt, or use 1/2 tablespoon of lemon juice mixed with 1/2 cup of milk and leave to curdle for 5 minutes before using.Cake will keep, well-wrapped at room temperature, for up to a day. It will keep in the fridge for up to 5 days.
Find this and more delicious recipes at www.katiebirdbakes.com!