Line the bottom of a springform pan (26 cm Ø) with baking paper. Grease the rim of the mould. For the cookie dough break chocolate into pieces. Melt in a hot water bath. Cream 125 g butter, sugar and 1 pinch of salt with the whisk of the mixer.
Stir in egg first, then chocolate. Mix flour and baking powder. Stir into the chocolate mixture. Press the dough in the mould to a smooth base.
Break the chocolate into pieces for the fondant. Melt with butter in a hot water bath. Allow to cool briefly, stir in flour. Stir eggs and sugar until creamy. Fold in the chocolate mixture.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Heat marshmallow cream in a water bath until it is easier to spread (alternatively, cut the chocolate kisses with wafer crosswise into 3 slices).
Spread on the cookie base. Add the fondant. Bake in a hot oven for about 25 minutes. (The cake is still soft. If you prefer it baked through, bake for about 10 minutes longer). Take out and let it cool down.