Remove the Hazelnut Skins
In a 3 quart or larger saucepan, bring the 3 cups/710 ml water to a boil. Remove from the heat and stir in the baking soda. The water will bubble furiously. Add the hazelnuts and return the pan to the heat. Boil the nuts for 3 minutes. The water will turn a deep maroon from the color of the skins. Test a nut by running it under cold water. The skin should slip off easily. If it does not, boil for a couple of minutes longer. Pour the nuts into a colander and rinse them under cold running water. Place several nuts in a bowl of cold water. Slide off the skins and place the nuts on a clean dish towel to dry. As necessary, empty the bowl, refill with cold water, and add more nuts.
Toast the Hazelnuts
Set the hazelnuts on the baking sheet and bake for about 20 minutes, or until lightly browned. Stir once or twice to ensure even toasting and avoid overbrowning. Set the baking sheet on a wire rack. If not using the Silpat, transfer the hazelnuts to the second baking sheet.
Make the Praline Powder
In a small saucepan, preferably nonstick, stir together the granulated sugar and the ⅓ cup/79 ml water and heat, stirring constantly, until the sugar dissolves and comes to a boil. Continue boiling, without stirring, until the syrup caramelizes to deep amber (an instant-read thermometer should read about 370°F/188°C or a few degrees lower because the temperature will continue to rise).
Immediately remove the pan from the heat and pour the caramel over the hazelnuts. Let it harden completely, about 15 to 30 minutes.
Remove the praline from the sheet and break it into several pieces. In a food processor, pulse the praline until it is powder fine.