To prepare the ice cream, heat 1 inch of water in the bottom half of a double boiler over medium-high heat. Place the semi-sweet and unsweetened chocolates, coffee, and instant espresso powder in the top half of the double boiler. Tightly cover the top with film wrap. Heat for 6 to 8 >>> minutes, then remove from the heat and stir until smooth. Set aside until needed. Heat the heavy cream, half-and-half, and 1/2 cup granulated sugar in a 2 1/2-qt saucepan over medium-high heat. Stir to dissolve the sugar. Bring the cream to a boil.

While the cream is heating, place the egg yolks and the remaining 1/2 cup sugar in the bowl of an electric mixer fitted with a paddle. Beat the eggs on high for 2 to 2 1/2 minutes. Scrape down the sides of the bowl.

Contine to beat on high until slightly thickened and lemon-colored, about 2 1/2 to 3 minutes. (At this point, the cream should be boiling. If not, adjust the mixer to low and continue to mix until the cream boils. If this is not done, the egg yolks will develop undesirable lumps. Pour the boiling cream into the beaten egg yolks and whisk to combine. Return to the saucepan and heat over medium-high heat, stirring constantly, until the cream reaches a temperature of 185 degrees, 2 to 4 minutes. Remove from the heat and transfer to a stainless steel bowl. Add the melted chocolate mixture and whisk to combine. Cool the mixture in an ice-water bath to a temperature for 40 to 45 degrees, about 20 minutes.

Freeze in an ice-cream freezer following the manufacturer's instructions. Transfer the semi-frozen ice cream to a plastic container. Using a rubber spatula, fold in the frozen, chopped pralines. Securely cover the container, then place in the freezer for several hours before serving. Serve within 3 days. Yields 1 1/2 quarts.

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