Preheat oven to 350 degrees F.: When you turn on the oven, you should notice a faint warm scent minutes later as it approaches temperature. Preheating ensures the cakes begin baking immediately, which helps them rise properly. A common error is sliding batter into a cold oven, which yields dense, underbaked centers and uneven browning. Use an oven thermometer if your oven runs hot or cool, and place the rack in the center for even heat circulation.
Cream butter and sugar together until light and fluffy. Add egg and mix until well combined. Add in vanilla and yogurt and stir. Mix dry ingredients together and add to batter, only mixing until just combined. Fold in apple chunks.: As you cream the butter and brown sugar , watch for a pale, airy texture, this signals enough air has been incorporated for lift. The sound will change from a heavy scrape to a lighter cream as sugar dissolves into the butter. After adding the egg , stop mixing the moment it looks unified, because overworking develops gluten and softens the crumb undesirably. Stir in the vanilla and plain Greek yogurt , which will slightly loosen the batter and add tang. When combining the dry mix of whole wheat pastry flour , all purpose flour , baking powder , salt , and spices, whisk to disperse the leavener and spices evenly. Fold the dry mix into the wet with gentle motions, leaving a few streaks rather than striving for over-perfection, this prevents toughness. The diced apple should be folded last so the pieces stay intact and distributed, giving bursts of texture. A typical mistake here is overmixing after the flour goes in, which tightens the crumb and reduces tenderness.
Spray 2 ramekins {I used the 6 ounce, 3 1/2 inch} with non-stick spray and pour batter into them equally. Bake for 30-35 minutes or until inserted toothpick comes out clean.: Greasing the ramekins prevents sticking and helps the edges brown evenly. Pouring batter equally ensures uniform bake times; if one ramekin is fuller, it may need more time and risk drying out. While baking you will see the batter puff and edges set, and a gentle aroma of cinnamon and baked apple will rise. The surface should turn lightly golden and spring back when touched. Insert a toothpick into the center at the earliest time, and if it comes out with a few moist crumbs, it is done; wet batter means more time is needed. Avoid opening the oven door frequently, as temperature shifts can cause sinking in the middle.
For the brown sugar topping, I simply combined 1/2 tablespoon flour, 1 tablespoon brown sugar, 1/4 teaspoon cinnamon, a pinch of salt and 2 teaspoons of melted butter and then crumbled it over top before baking. For an optional glaze on top, combine 3-4 tablespoons of powdered sugar and a drop or two of milk (literally a drop) and whisk until a glaze forms.: The topping ingredients should be mixed until crumbly so they create tiny pockets of caramelized sugar on the cake surface. The melted butter helps bind the crumble so it browns but does not dissolve into the batter. When you sprinkle it on, the contrast between crunchy bits and tender crumb is immediate after baking. For the glaze, whisk the powdered sugar with just a smidge of milk to a thick but pourable consistency; too much milk makes the glaze runny and masks the cake's texture. Apply glaze sparingly after the cakes have cooled slightly to avoid melting it away. A mistake here is adding the topping too late in the bake, which can keep it from crisping, or adding the glaze while the cake is piping hot and causing it to soak in rather than coat.