Make the streusel: Mix all dry ingredients in a bowl (brown sugar, sugar, flour, and cinnamon). Add the cold chunked butter and "cut" in using 2 steak knives or a food processor. Once butter is mixed in, but still crumbly, set in refrigerator until ready to use.
Make the cupcakes: Mix the Yellow cake mix with ingredients listed on the box (water, oil, and eggs). Add the vanilla, cinnamon, and nutmeg. Divide batter evenly into 24 cupcake liners.
Add the streusel on top of the cupcake batter, (approx. 1 Tbsp. each). Bake cupcakes according to instructions on cake mix box. Cool cupcakes completely before frosting them.
Make the frosting: Beat the softened butter on medium-high until smooth and creamy. Add half of the confectioners' sugar, half of the heavy cream, scraping the sides often. Add the rest of the ingredients and allow to beat on high for a few minutes to thoroughly blend the ingredients and to let the frosting get light and fluffy.
Pipe or spread your maple buttercream frosting on top of your cooled cupcakes for the full French Toast Cupcake experience. Enjoy!