A buttery and warmly spiced apple cinnamon coffee cake is made with browned butter, brown sugar cinnamon ribbon, tender apples, cinnamon crumb topping, and a thick brown butter vanilla glaze.
Add the butter to a light colored saucepan over medium heat. Let it melt completely, then continue cooking as it begins to bubble, and foam. Then it will have a crackle sound. Stir or swirl the pan frequently so the milk solids brown evenly. After several minutes, the foam will begin to settle and you’ll see golden brown specks forming on the bottom of the pan. Once the butter smells nutty, the milk solids are a deep golden amber, and you don't hear that crackle sound anymore, immediately remove it from the heat so it doesn’t burn. Pour the butter, including all of the browned bits, into heat safe bowls. Let the butter for the cake batter cool for about 10 to 15 minutes, or until cool to the touch. The butter for the crumb topping can be used warm, and the butter for the glaze should cool for about 5 minutes.
Combine the brown sugar, granulated sugar, flour, cinnamon, salt, and nutmeg, if using. Add the browned butter and mix with a fork or your fingers until crumbs form. Keep a mixture of small crumbs and larger clusters. Refrigerate while you prepare the cake.
In the same saucepan or a skillet, melt 2 tablespoons of butter. Add the diced apples, brown sugar, cinnamon, lemon juice, and salt. Cook over medium heat for 4 to 6 minutes, stirring occasionally, until the apples begin to soften but still hold their shape. Remove from the heat and stir in the vanilla. Sprinkle the 1 TBSP of flour over the apples and stir until evenly coated. Let the apples cool completely.
Mix the brown sugar, cinnamon, and salt together in a small bowl. Set aside.
Preheat the oven to 350°F. Grease and line an 8 x 8 inch metal baking pan with parchment paper. Whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg in a medium bowl. In a large bowl, whisk the cooled browned butter and granulated and brown sugar together. The mixture will look slightly grainy. Add the eggs one at a time, then whisk in the vanilla, sour cream, and milk until smooth. Add the dry ingredients and mix just until no streaks of flour remain. Do not overmix.
Spread a little more than half of the batter into the prepared pan. Sprinkle all the cinnamon ribbon evenly over the top, leaving a small border around the edges. Dollop the remaining batter over the cinnamon layer and gently spread it over the top. Spoon the cooled apples evenly over the batter and lightly press them into the surface. Scatter the crumb topping over the apples, with your fingers breaking some into smaller crumbs and pinching others into larger clusters.
Bake for 50 to 60 minutes, or until the center is set, the crumb is golden brown, and a toothpick inserted into a cake section comes out with a few moist crumbs. Avoid testing directly through an apple. If the topping browns too quickly, loosely tent the cake with foil. Cool in the pan for 30 to 45 minutes before glazing.
Brown the tablespoon of butter and let it cool for about 5 minutes. Whisk in the powdered sugar, vanilla, salt, and 1 tablespoon of milk. Add more milk, 1 teaspoon at a time, until thick but pourable. Drizzle over the cooled coffee cake and let the glaze set before slicing.
You want a golden color, not burnt, like toasted hazelnut, not coffee ash.
Measuring flour properly makes a big difference in how your baked recipes turn out. For best results, use a kitchen scale and measure in grams. This ensures consistency and prevents adding too much flour, which can make this cake dry or dense.
If you don’t have a kitchen scale:
Too much flour is one of the most common reasons baked recipes can become dry and cakey