Sticky Toffee Pudding Thumbprints

Place dates in a small heatproof bowl. Pour in enough boiling water to cover, then stir in baking soda, and soak dates for 10 minutes.

Meanwhile, place 1 cup butter in a light colored saucepan over medium-low heat. Cook, stirring constantly, until butter is browned, 5 to 10 minutes. Pour browned butter into a large mixing bowl and allow to cool for 10 minutes.

Drain dates; add to a food processor or blender, and pulse until mixture is thick and paste-like, with few large chunks of date remaining.

To the browned butter, add brown sugar, molasses, vanilla, espresso powder, salt, nutmeg, cloves, and allspice; stir to combine. Add in egg and stir until well incorporated, about 1 minute. Add in dates, and stir until thoroughly combined. Add in 1 1/2 cups flour, cornstarch, and baking powder and mix until just combined. Add in remaining 1 cup flour and stir until just incorporated.

Pour 1/2 cup white sugar in a small bowl.

Scoop out 1 tablespoon dough and roll into a ball; roll in sugar until completely coated. Dough will still be so somewhat soft, but should still roll into a ball. Using your thumb, or the handle of a wooden spoon, spatula, or small whisk, make an indentation in the center, about 1/2-inch wide. Place cookies on a plate. Repeat this process with remaining dough. Refrigerate until chilled, at least 30 minutes.

Preheat the oven to 350 degrees F (180 degrees C). Line baking sheets with silicone baking mats or parchment paper. Arrange chilled cookies 1 inch apart on prepared baking sheets. 

Bake until cookies begin to puff, and small cracks begin to appear, 12 to 14 minutes. Remove cookies from oven, and immediately use the handle of a wooden spoon or spatula to press the center of cookies back down. Cool cookies on the baking sheet for 5 minutes, then remove to a wire rack to cool completely.

For toffee sauce, place brown sugar, butter, water, molasses, and salt in a heavy-bottomed sauce pot over medium-low heat. Cook, stirring constantly, until mixture is boiling. Reduce heat to low and boil for 2 minutes, stirring constantly. Carefully pour in cream (mixture will sputter), while continuing to stir. Return to a boil, and cook for 5 minutes, stirring constantly. Remove from heat; stir in vanilla and lemon juice. Pour into a heatproof glass measuring cup with a pour spout. Allow toffee sauce to cool until slightly thickened, stirring occasionally, 15 to 20 minutes.

Carefully pour toffee sauce into center of each cookie, taking care to not overfill. Alternatively, you can spoon the sauce into the cookies instead of pouring from a measuring cup. Toffee sauce will thicken as it cools. Store cookies in a single layer in an airtight container.