Heat oven to 350 degrees. Use butter to grease 6 half-cup muffin molds or similar, or use smaller molds. Dust molds with cocoa and sprinkle the sesame seeds in the bottom of each. Sift together remaining 1/3 cup cocoa with the flour and a pinch of salt. Set aside.
Melt chocolate and remaining 10 tablespoons butter in a heatproof bowl over simmering water, or in a small, heavy saucepan on very low heat. (Watch carefully if using a saucepan.) When almost completely melted, remove from heat and stir until smooth. Set aside.
Using a whisk or electric beater, whisk eggs and sugar together until pale and fluffy, about 5 minutes. Stir a little of this mixture into the tahini to lighten it, then stir the tahini into the egg mixture. Stir in the cooled chocolate mixture. Fold in the cocoa mixture.
Pour the batter into the molds and set molds on a baking sheet. Place in the oven and bake 12 to 14 minutes, less for smaller molds. A skewer or toothpick inserted in the middle should not come out clean.
Cool 20 minutes or more before unmolding. Dust tops with lime zest and serve with crème fraîche or ice cream.