• 1 cup high fiber flour
• 1 tsp baking powder
• 1 tsp baking soda
• 1 tsp Diamond Crystal or ½ tsp Morton kosher salt, plus more
• ½ tsp ground cinnamon (optional)
• ½ cup (1 stick) unsalted butter, divided
• 1 cup light brown sugar (monk fruit), divided
• ½ cup granulated sugar (allulose)
• 1 tsp vanilla extract
• ½ tsp salt
• ½ tsp ground cinnamon
• 2 large eggs
• 3 large Granny Smith apples, peeled, grated on the
  large holes of a box grater
• ¼ cup heavy cream

Preheat oven to 350°. Line bottom and sides of an 8×8” baking dish with parchment paper. Whisk flour, baking powder, baking soda, salt, ground cinnamon (if using) in a medium bowl.

Melt 5 TBS of the butter in a small skillet over low heat, then pour into a large bowl, reserve skillet. Add ½ cup brown sugar, granulated sugar, and vanilla extract to large bowl and whisk to combine. Whisk in eggs. Add dry ingredients and whisk until mostly combined. 

Add peeled and grated apples and whisk until apples are distributed and no spots of dry flour remain. Scrape batter into prepared baking dish; smooth top.

Bake cake until a tester inserted into the center comes out clean, 30–35 minutes. Transfer to a wire rack and let cake cool in pan. Invert cake onto a platter and remove parchment paper. (Turning the cake bottom side up ensures a nice flat surface for glazing.)

Just before serving, melt remaining 3 TBS butter in reserved skillet over medium heat; cook, stirring constantly, until milk solids are deep brown, about 3 minutes. Add heavy cream, a pinch of salt, and remaining ½ cup light brown sugar. Stirring constantly, bring mixture to a boil and cook until slightly thickened and sugar is melted, 1–2 minutes. Pour glaze over cake and spread to edges with an offset spatula.