Peel and cut the apples into medium-sized cubes, then toss them with the cinnamon.

Place the apples in a skillet and cook over medium-low heat with a lid for about 10–15 minutes, until softened. You don't need to add water, as the apples will release their own juices. If there is too much liquid, increase the heat to let it evaporate, being careful not to let the apples stick or burn.

Remove from heat and let cool.

In a bowl, sift together the oat flour, baking powder, and vanilla.

In another bowl, use an electric mixer to beat the eggs with the brown sugar.

When the mixture is pale and fluffy (it should at least double or triple in volume), reduce the mixer speed.

Add the oil and milk, continuing to mix.

Gradually add the sifted dry ingredients until the batter is smooth.

Add the pinch of salt and turn off the mixer.

Gently fold in the apple cubes with a spatula, mixing from the bottom up. (If the batter seems too dry, you can add a little more milk.)

Pour the batter into a greased and floured Bundt pan (or use nonstick spray). Note: To level the batter, never bang the pan—gently shake it back and forth or side to side instead. (Banging the pan will bring air bubbles to the surface and the cake may not rise properly.)

Bake in the center of a preheated oven at 350°F (180°C) for about 45 minutes.

When done, turn off the oven and leave the door slightly open for 10–15 minutes (this helps prevent the cake from collapsing due to a sudden temperature change).

Let the cake cool completely, dust with powdered sugar, and serve.