Mads are one of my specialties and I use a battle-tested variation of Julia Child's Madeleines de Commercy, which I've included below along with tips and variations and some troubleshooting for the recipe/technique you currently have. Hope it helps, and let me know if you have any questions!
Troubleshooting Original Method/Recipe
-Traditional mads don't use chemical leaveners. It's not the greatest sin to use a pinch, but try to reserve it for times when add-ins like mini-chips make your batter particularly heavy.
-Batter is too liquid. A mad batter with that amount of flour can't handle 2 tablespoons of lemon juice and a teaspoon of vanilla and keep its structural integrity. Increase dry flavorants (peel, spices etc) as needed, but keep your added liquid to no more than a tablespoon.
-The recipe you're using calls for the Whisked Sponge method, which puts a lot of volume into the batter creating an egg foam. Great method, but totally the wrong one for mads, which use the Melting method. When you use the WS method, especially alongside chemical leaveners, you're going to get a sponge or butter cake, not a mad.
-The cups are overfilled. Try scaling back ~20%
-Your pans are lovely, but I'm not sure you're getting the best mads you can with them. See recc. below.
-Oven is slightly too cool. Baking little cakes for a long time, especially when you're using leaveners and an egg foam, gives them a chance to rise *and* fall. If you want that hump you want to bake it hotter and faster.
Okay, now that I've tackled that; let's get on to the good stuff: the recipe, variations and tips
Madelienes de Commercy:
2 eggs, room temperature
120g (1C) flour
140g (2/3C) granulated sugar
140g (~1 1/4 stick) butter
generous pinch of salt
Flavorants (vanilla, lemon peel) as desired, liquid not to exceed 5ml/1 tsp
Brown butter until nutty but not burnt. Cool until room temperature but still liquid.
In separate bowl, beat together eggs and dry ingredients until well combined.
Beat in butter and flavorants if using. Chill at least one hour but do not freeze.
When ready to bake, spoon but do not smooth batter into a prepared and frozen mad pan, taking care not to overfill.
Bake at 375F/190C for ~14min until edges are brown and the middle is humped.
Turn out immediately and cool shell-side down. Makes ~24
-Replace 50g of flour with fine almond flour & use almond extract
-Add 1 tbs minced fresh rosemary to butter while browning, increase salt to 1/2 tsp (this is my signature mad)
-Get the right pan. I swear by the Fox Run nonstick mad pan.
-Freeze your empty mad pans after buttering/flouring but before baking. Freeze again between batches.
-Let your batter rest. An hour is good, three hours is better, overnight is best.
-Don't spread your batter into the molds; just drop in a bit of batter like you would a drop cookie.
-Underfill rather than overfill the mold so not to ruin the shape