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First make the cake: Preheat the oven to 325 F. Spray a 10-12 cup (with a smaller pan, you may get the rounded effect on the bottom of the cake that I got, from it rising over the edge of the pan) bundt pan with nonstick baking spray. Set aside.
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In a large bowl, whisk together the flour, baking powder, baking soda and salt. Set aside.
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In a stand mixer fitted with a paddle attachment, beat the butter until creamy, about 3 minutes on medium speed. Add the sugar in several additions, scraping down the bowl inbetween, and beat until the mixture is pale and fluffy.
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Add the eggs, one at a time, scraping down the bowl after each addition. Add the vanilla extract and paste and beat to incorporate.
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Whisk together the Tuaca and yogurt.
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Add the flour mixture and the yogurt mixture to the batter, mixing on low. Add in 4 and 3 batches, respectively, beginning and ending with the flour mixture.
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Beat on medium-high speed for 20 seconds.
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Scrape the batter (it will be stiff) into the prepared cake pan. Smooth the top with a spatula.
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Bake the cake for 50-60 minutes. The cake is done when it is golden brown, pulling away from the edges of the pan, and a cake tester inserted into the center of the cake comes out clean or with just a few crumbs attached.
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Remove the cake from the oven and allow it to cool in the pan for 10 minutes. Invert the cake out onto a cooling rack lined with parchment paper. Let cool for one hour.
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At the 45 minute mark, make the butterscotch glaze. Melt 1 cup of sugar and the agave syrup over medium heat in a heavy saucepan. Stir until dissolved. When the mixture is clear, stop stirring and let it boil untouched.
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Cook the sugar until it is deep amber (if you have a candy thermometer, about 300 F).
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Slowly drizzle the water into the caramel--stand back as it will bubble up. Stir to incorporate.
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Turn the heat off and add the butter. Whisk to incorporate until the butter is melted and mixed in. Mix in the salt.
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You will have more caramel than you need. Drizzle as much of the butterscotch glaze over the cake as you want--I used about half of mine. Try to cover all of the outside (the shape of the cake will affect this). While the glaze is still warm and sticky, sprinkle the outside of the cake with coarse sea salt.
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Let the glaze set and the cake cool completely before slicing.