1. Preheat your oven to 350 degrees F and set your oven rack to the middle position. Grease a 6-inch cake pan (I use a 6-inch spring-form pan, which I purchased right in my usual supermarket).

To brown the butter

1. Place the butter in a small saucepan set over medium-low heat. Stirring frequently, cook until the butter melts, then continue cooking and stirring until the foaming subsides and the solids on the bottom of the pot begin to brown. Once they’ve browned, immediately pour the butter into a heatproof bowl to stop the cooking (we don’t want burnt butter!). Place the bowl of butter into the fridge to allow it to cool and solidify before using in the cake.

To make the brown butter apple cake:

1. Scrape the solidified brown butter into a mixing bowl, and add the granulated sugar. Blend on med speed until light and fluffy. Add the egg, sour cream (or yogurt), and vanilla and blend until smooth.
2. In a small bowl, whisk together the flour, baking powder and soda, the salt, cinnamon, nutmeg, and allspice. Add the dry ingredients to the browned butter mixture and blend just until combined. Add the diced apple to the batter and gently fold in to incorporate. Pour the batter into the greased pan and top generously with the streusel (see below for streusel directions). You will probably have extra streusel leftover, and that’s okay! You can save it for a later use.
3. Bake the cake until golden brown; about 35 to 45 minutes (maybe longer; just insert a cake tester or knife into the center of the cake to test for doneness).
4. Allow the cake to cool almost completely before removing it from the pan and dusting with confectioner’s sugar.

For the streusel:

1. Combine the flour, cinnamon, and sugars in a medium bowl. Add the butter and work it in until it is evenly incorporated. I like to use a pastry blender or fork for this, and then I typically switch to my hands because it’s delightful. Once the butter is evenly incorporated, add the cereal and mix gently to evenly combine.