In the 3-quart heavy saucepan cook ¼ cup (44 g) sugar, over moderate heat until it begins to melt and cook. Don’t stir the sugar until it has begun melting, then stir with a fork until melted completely and deep golden brown. Don’t be tempted to walk away; you need to keep a close eye on caramelizing sugar. (You can add 1 tablespoon of corn syrup to the sugar once it starts melting to help prevent crystallization, but that’s optional.)
Add the hot milk very, very slowly. Start with just 2 ounces (60 ml) and pour very gently, whisking. The hot mixture may bloom up the side of the pan, so do this very carefully. Add a little more and continue whisking until all the milk is in the pan. If the caramel hardened, continue cooking until it has melted. Whisk in the sifted cocoa until combined well. Keep the mixture warm. (Alternate directions for the more cautious: After caramelizing the sugar, remove pan from heat and dip pan briefly into a bowl of ice water to stop cooking. Caramel will harden. Cool pan about 5 minutes and return to heat. Add milk and cook over moderate heat, whisking, until caramel is melted.)
In a metal bowl set over a saucepan of barely simmering water (or a double boiler), melt the bittersweet chocolate, stirring, and remove from heat. (You can also do this in the microwave in a glass bowl. Heat 30 seconds at a time until almost all the chocolate is melted, then stir in the remaining.)
In the bowl of the stand mixer, beat the egg yolks with remaining ¾ cup (131 g) sugar until thick and pale. Whisk in caramel mixture and chocolate in streams until combined. Pour custard into the 3-quart heavy saucepan and cook over moderately low heat, stirring constantly, until the candy thermometer registers 140° F/60° C. Stir and cook custard, 4 minutes more. Do not allow it to boil. Remove the pan from heat.
Cool the custard to room temperature, than overnight in the refrigerator. Freeze in an ice-cream maker according to manufacturers instructions. Store churned gelato in a plastic freezer container with tight-fitting lid.
In the heavy skillet, cook the dry sugar over medium heat until the edges start to melt. Do not stir. As the sugar melts, you can swirl the pan a little to get the melted sugar moving around the pan. Continue cooking until the melted sugar turns a deep golden brown. Again, stay with the sugar while it cooks. (You can add 1 tablespoon of corn syrup to the sugar once it starts melting to help prevent crystallization, but that’s optional.)
Scatter the hazelnuts onto the sheet pan lined with the silicone mat and pour the hot melted sugar over the nuts. Allow to cool completely before breaking apart the praline.
Serve scoops of gelato with shards of praline on top.