Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/block_content/templates/block-content-add-list.html.twig

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+{#
+/**
+ * @file
+ * Default theme implementation to present a list of custom block types.
+ *
+ * Available variables:
+ * - types: A collection of all the available custom block types.
+ *   Each block type contains the following:
+ *   - link: A link to add a block of this type.
+ *   - description: A description of this custom block type.
+ *
+ * @see template_preprocess_block_content_add_list()
+ *
+ * @ingroup themeable
+ */
+#}
+{% spaceless %}
+  <dl>
+    {% for type in types %}
+      <dt>{{ type.link }}</dt>
+      <dd>{{ type.description }}</dd>
+    {% endfor %}
+  </dl>
+{% endspaceless %}